I can think of one example that I've encountered that seemed pretty subjective. In vanilla there is a clear progression of dragons, normal -> blood -> frost -> elder -> ancient. Each dragon occurs twice with two different breath attacks, except the frost dragon which obviously uses frost breath for both of its LeveledActor entries. This distribution allows the player to encounter every visibly distinct species with equal likelihood, assuming the player/cell/spawn is of sufficient level.
But after USLEEP, the second copy of Frost dragon has been replaced by a weakened version of the Elder dragon. As a result, you are 3 times more likely to encounter an Elder dragon than a Frost dragon. It's made even worse by the fact that Beth used the same mesh for the Ancient dragon, presumably as a last minute addition. So you are 5 times more likely to encounter that mesh than the Frost mesh. USLEEP's reasoning seems to be that players care more about which breath a dragon is using than its appearance/name. Whether or not that's true seems like a matter of opinion. AFAIK there is no associated bug being fixed by this change. The other possible explanation is that this was intended to restore the rank 3 fire dragon that had been cut from the game, but that would mean this should be part of Cutting Room Floor rather than USLEEP. Especially since the dragon doesn't have it's own model.