Not counting RBY and GSC (Because it'd be sorta unfair imo to compare them as games with more recent ones, they just did not age well)
Diamond and Pearl (Not counting Platinum) is seriously one of the worst games of the franchise, for 2 main reasons: Everything is slow and the dex for no reason is severely limited to just 151 mons overall.
The first point just damages gameplay experience a little, it's annoying but you can at least get around. Doesn't make it fun tho, a lot of battles take forever to end just because everything from animations to HP dropping were slow. Also surfing was very very slow as well. Aside from that there were arbitrary ways to slow you down too, with Fog doing absolutely nothing interesting other than make you miss more which just makes battles drag on for longer, those mud piles that do nothing other than get you stuck til you mash buttons to get out, and the snow route. None of these things make for an enjoyable experience imo, specially since the last 3 don't add anything other than slowing you down and wasting your time
The dex limitation tho, is a huge flaw. Sinnoh did this really cool thing where it added a lot of evolutions to older mons. So why the actual hell does Diamond and Pearl just... not have them at all? I don't see the point in adding evos to Magmar, Togetic, Lickitung, etc if most of those can't even be caught in game. A lot of people joke about how their DP teams were mostly similar, but that's less so because of people liking the mons available, moreso because DP barely gives you options (Just compare people's Platinum teams instead - they are a lot more varied because the game allows them to have variety in the first place). Not only that but the pokemon that are available are still not all very accessible. Not counting the few ones that are locked behind the postgame, you still have some pokemon locked behind specific days or times, ones that require an in-game event to be completed, ones that you can only get by a very late game egg that even after you hatch you'd have to get friendship up and level it all the way from lvl 1, mons locked behing that terrible terrible marsh, mons locked in that weird unown cave and lastly the ones on the honey trees which are not very user-friendly to get when the trees themselves take a pretty long while to spawn a pokemon, and you can't even be sure if the one that spawns is the one you want (Not even mentioning Munchlax here)
Not only as this affects the main appeal of pokemon - everyone getting their own personal team depending on what mons they like most - it also affects boss design. A lot of gym leaders and E4 in DP have pretty boring teams because the pokemon variety is very small and doesn't allow them to have many choices since they are already type-locked. Sure you have the classic Flint meme of using non-fire mons in his team, but there's also Volkner with half his team not being Electric and Candice with medicham. And sure type variety is nice, but when that isn't a game design choice but rather having to work around a previously stablished dex, it kinda showcases that the dex itself is not good.
Lastly there's just some personal complaints I have with the games itself. The two above are objective flaws (Still up to you wether they are flaws that bother you or not tho), but the rest of this paragraph is just my opinion on a few things from the games. I don't think Barry nor Cyrus have any meaningful arc in DP; most of Cyrus' good moments are in the Distortion World and DP doesn't have that. Meanwhile Barry just... doesn't really go through any development, really. My other complaint is that the level curve at the end of the game is absolutely horrible. At the victory road, the highest level you'll face is 49. Even then, the trainers in the Victory Road with a high level mon usually only have a single one. Then from that you go straight into the league where there's 5 mons ranging from 53 to 57, only getting worse at each member til the last one starts at 59 and ends at 63. And after that is Cynthia, who starts already at 60 and has her ace at 66. Sure we all have heard the jokes about Cynthia being difficult - and to an extent that is still partially credit to her team being very well balanced - but part of that also comes from the fact she will always be around 10 or more levels higher than you. Don't get me wrong, I do like difficulty in Pokemon games, but this just isn't a very good game design, specially when there is absolutely no good and reliable way of grinding for levels at that point in the game. You are basically forced to face trainers who will for the most part have higher level than you and there isn't much you can do about it. It's a terrible way to cap off the game and a sincerely terrible way of designing boss fights
Platinum really saved Sinnoh from most of this by a few speed increases in some areas and most importantly by having an actually good dex. If it hadn't been for Platinum, I'd genuinely think Gen 4 would be the worst gen. As it stands right now tho, at least 1 of the 3 Sinnoh games is pretty enjoyable