I've started with gen 4 and have been playing since I was around 3 or so, but looking at the game while trying to take off them nostalgia goggles, I'd have to give it either a 7-7.5/10 objectively. Yes, there are problems here and there, but there are also some good things too, which a lot of people want to skim over. But the cons, cuz this game is by no means perfect, should probably be listed first.
By the time I finished this, I realized I put A LOT, so the tl;dr is just - 7.5/10. Lots of problems that could've easily been avoided, but still adds some nice things and is still very enjoyable. Platinum is overall better, but both have good things that the other doesn't. I don't see why you shouldn't just play both. Overall, ILCA did a pretty good job, considering them being new to this and probably being limited in what they can actually do.
Cons:
-Removal of Underground Secret Base decoration. If ILCA wanted us to have statues, then we could've easily just had a statue room or tiny display cases that give small buffs.
-Exp All/Share forced on the party. This has been hammered into the ground, and for good reason. There is no good arguable reason on why the exp share is forced and gives so much. It's not split correctly, and you technically get more exp than normal, with the rest being split. It's very easy to overlevel in this game if you're not careful or don't use more than 6 Pokemon for your playthrough. This. Mechanic. Isn't. Good. This game existing in gen 8 messed things up.
-Control. While things have been getting somewhat better with updates, the 1:1 take of Sinnoh left no changes to obstacles and map design to make the tight spaces easier to go past without using the D-Pad, and there can still be some bumping problems there too.
-Bugs. This is by far one of the buggiest mainline Pokemon games I've seen. It's no Gen 1, but there are some issues. My 2nd playthrough had 1 crash, and I've had some Grand Underground problems with random sparks and Pokemon getting stuck. While I personally haven't dealt with a lot of actually harmful issues other than the 1 crash, I know a lot of other people had way more problems. Also, that 7th gym softlock shouldn't've existed, even if you really shouldn't be in that situation in the first place. ILCA hasn't really been the lead in development for games(Home is an App, and for the other games they've been in, they weren't majorly involved)
-Platinum content. This is one of the other glaring problems with this game. Platinum is objectively better and fixed almost every problem Sinnoh has. For whatever reason, a good chunk of that just isn't here. Some pokemon are catchable. Rotom forms are available. Then you have the rest of the dex. The original DP dex isn't good, and BDSP only slightly fixed that. Item placement for better progression, dex expansion with evolutions, and more have been left out. There are SO many people I've seen get the ":(" because the Razor Fang for Gliscor is postgame. I don't know if it was Masuda that thought it should be like this because he directed DP and not Pt, or ILCA REALLY wanted the games to be fundamentally flawed, but yeah.
-Single Use TMs. I personally don't have a problem with it. If you just play in the Grand Underground and talk to the hikers, then you can trade spheres for TMs. All of them can be obtained more than once. I know that a number of people still dislike it, and some don't want to have to do that, so I'm putting it as a con.
-Difficulty. The affection mechanic and exp share messed up a lot of things, and there was nothing done to actually make the game scale with it. Levels and Pokemon aren't changed, with the only real challenges coming in the E4 and some gym leaders if you don't have a good team for it. Maylene messed some people up with Lucario. Roark caught some people off guard with Cranidos. Every other gym leader is... there? Of course, Pokemon isn't really meant to be that hard, and it's "was always easy", but a lot of the fun comes from building your team, connecting with the mons, and getting success from playing the game right, not getting your ass beat 6 times.
-Poketch issues. There's literally no reason to have the Poketch have 1 button again. There's being faithful, and then there's just being plain stupid. It also doesn't help that the touch screen only works with badge cleaning it seems. It may feel clunky and slow to use the Poketch at times due to you having to press the R button, find the app, use the thing, then put it back. Having the right stick always have a pointer on it or something could've helped...
-Generation 8 JUNK. Forced EXP share is one thing, but there are also other problems like removed moves. Honestly, if this remake came out during Gen 7 or something, we might've been able to dodge some of these flaws in design and structure.
Pros:
-Music. The music is pretty much perfect, no argument. Only a couple of songs may not hit as well as the originals, but this is just good. Everything has been modernized and has just been done well. Sinnoh has one of the better soundtracks overall and is up there with Hoenn and Johto. Yes, Unova is also good with the OST, but also has some misses.
-The graphics. You can say you don't like the chibis, and that's fine. It's not bad though. I don't get how there can be so much discourse over this artstyle, because it's fine at worst, and great at best. The game is vibrant and looks great in most areas. Yes, the chibis can take away the seriousness of many characters and scenes, but they also add a loveable charm, cuteness, and entertainment from just looking at them. The only thing I can say is that the game is sometimes too bright, and sometimes Pokemon just straight up glow or reflect light when they shouldn't.
-The Grand Underground. God, you can hear about people just spending hours down here, and there's a good reason why. Seeing a bunch of people run around feels great. Getting blue sparks to mine with them helps. Mining in general is addicting and rewarding, leading to a lot of goodies. Hideaways have extra goodies, gives mons that are a good level, gives MORE mons that expands the dex or give earlier access, AND they have egg moves, making it easier to get good movesets(Looking at you, Sneasel). The Diglett "Lightstone" mechanic is great too and helps with shiny hunting and statue collecting, which helps you encounter certain Pokemon.
-It's Sinnoh. Love it or hate it, the fact that it's literally just Sinnoh has a lot of reasons to have fun. The lore is cool, with lots of myths and stories. The routes/caves/etc, while sometimes a bit weird, always have something great in them and are WAY better than a lot of other regions, probably getting beat only by Hoenn(you can argue Unova too, but I personally don't think so.). While some evolutions and mons are misses, there are a LOT of hits, like Gliscor, Weavile, the Gible line, all 3 Starters, the BEST regional bird Staraptor, Bidoof, and a bunch of other ones that would take too long to list. For every needless Happiny, there's a cool Vespiquen or Gastrodon right there and then some.
-HM QOL. Sinnoh is notorious for having HM problems. You either need an HM Slave or HMs spread out across your team, which can lead to messed up movesets and problems. This game fixed it. The whole thing. It's all just a Poketch app. This reason is probably why BDSP can even stand with Platinum.
-Generation Updates. The game is faster than Sinnoh, which is notoriously slow(I don't mind but it is definitely painful at times). Fairy types, stat buffs, and some movepool upgrades make the games much more manageable. Remember when Shinx would get spark at like lv 13? Yeah.
-More Ball Capsule stuff. It's not even that big of a pro, but it's really cool. Gen 4 is the only gen to have this, so it being back with more stickers/seals is cool.
-Postgame. Say what you will about the Battle Frontier and Villa. I'm upset about it too, and that's in the cons category of "Platinum Content", but there's still a lot of good here. The entire island is still explorable, and Ramanas park is right there. A bunch of legendaries to catch and it incentivizes you to mine in the Grand Underground? Pretty damn cool. Dawn/Lucas can be battled. We also have Gym Leader + Barry rematches back.
-Contests. Not a lot of people really care about this, but they are simplified and faster. You may be happy or upset about it, but I'll mark it as a pro. This is more of a mixed thing.
-The Elite Four. Call it a difficulty spike, and I'll call it an amazing challenge that your entire adventure has led up to. Yes, there are ways to cheese the fights, like EVERY fight in the series, but Cynthia takes the crown and the cake as the strongest trainer in the series now. Leon got close, and then this game came out. Red is there too, but Cynthia's team is better with the rematches. The only thing that holds this back is the affection mechanic.
So yeah, there's good and bad. Every game has that. Pokemon fans may be a little bit more passionate and intense about their feelings towards a game, but that's because it's Pokemon. There's always so much to love and get attached to. Sinnoh will always be my favorite region, and while this game isn't the best thing that could've happened for remakes, it still has a lot to enjoy. I recommend it. If you don't like it, try looking into modding the game to get those sweet sweet fan-wanted changes. Take a look at Sigma Platinum, a BDSP mod. Wait a little while for more mods to come out, which will most likely fix a lot of the problems with this game.