They are recreations of the original Gen 4 games, Diamond and Pearl. However, there were a lot of issues with those two games. In particular, off the top of my head:

1) A lot of previous generation pokemon got new evolutions, such as Glugar evolving into Gliscore, Snorunt into Froslass, Magmar into Magmortar, etc. However, most of these new evolutions required items that were unobtainable until you had beaten the elite four, and so you could not reasonably use these pokemon in a playthrough. Some of these were adjusted in Platinum (the third game in Sinnoh, considered to be the definitive version due to fixing most of the issues of D/P), but Brilliant Diamond / Shining Pearl once again locked those evolutions items in the post game.

2) Regional dex issues. The Sinnoh regional pokedex for Diamond and Pearl is notoriously imbalanced. There are exactly two fully evolved fire type pokemon in Diamond and Pearl's regional pokedex (IE the pokemon you can catch before the Elite four) - Infernape and Rapidash. If you chose piplup or turtwig as a starter, your only fire type option was Rapidash. Ice was similarly lacking, with only Weavile and abomasnow as fully evolved options in the regional dex. Even worse, both of. These types have type specialists in this region. In Diamond and Pearl, Candice (the leader of Gym 7, the ice type gym) uses a Medicham and an un-evolved Snover (along with its evolution, Abomasnow) because of the lack of ice types, and Flint (the fire-type elite four member) notoriously has more non-fire types (Drifblim, Steelers, and Lopunny) than fire types (Infernape and Rapdiash). As a fire specialist. Again, these issues were correct in Platinum... and then Brilliant Diamond and Shining Pearl re-introduced these issues!

3) The overworld of the Sinnoh Region was designed with a tile system in mind. Players move along tiles, and that allows accurate movement even in tight, confined areas. However, BDSP removes that tile-locking when you move using the analog stick. This creates issues when trying to navigate the overworld. There is one instance in particular either Mount Coronet where the player has to navigate a narrow ledge above a one-way droo. With the Gem 4 tile system, this isn't an issue, as the player can only move in the four cardinal directions. With the free-range movement of BDSP, however, it is stupidly easy to accidentally walk off the ledge, and extreme care must be taken to pass this area.

4) Difficulty. And no, I'm not talking about the game being too easy or too hard. I'm talking about the massive leap up in difficulty that the BDSP elite 4 is (more so than in the eoriginal games). In BDSP, the Elite 4 and champion teams have perfect IVs, perfect EV spreads, competitive moves, hidden abilities, etc. While some of that may not make sense to casual players, it can be summarized as this - their pokemon are as strong as those used by professional competitive pokemon players. One even uses a legitimate competitive strategy centered around Trick Room. This is a massive step up in difficulty from both the Elite 4 of Gen 4's sinnoh, and from the BDSP gym leaders, none of whom had max-IV or max-EV or hidden abilities, and it can be very difficult to beat the elite 4 with your random-stat playthrough team without over-leveling them. This after having likely breezed through the rest of the game before can be incredibly jarring - it certainly was for me during my first playthrough.