The Revelation games definitely have less different types over. RE5 primarily focused on Majini. While RE5 did have those hallucinating bug things. Most of the enemies in my opinion felt a little uninspired in comparison to RE3 or even RE6. RE6 had some enemies that varied for each story. Some are more inspired, but not nescairly effective to the developers intention to the player. A good example of this was the Ustanak who Capcom liked to claim was one of the scariest RE creatures to date, something it wasn't.
The original RE and even RE0 was very varied. First I'm fighting Zombies, then I find infected flora and fauna. I'm discovering all these animals and even marine life that's been experimented on. It's constantly changing as I'm exploring, and those Leeches? AWESOME! RE5 started with fighting people and ended with fighting people with less alternative enemy types in the mix. RE4 had a range from the crazy Garrador's to Regenerators. I was also disappointed with the Chainsaw guy in RE5 who did not feel nearly as aggressive as the Dr. Slavador. Not to mention he repeated the same attacks so you can fight them without even moving with decent aim and the right rifle. Or just shot, punch, repeat.
Let's keep in mind that enemies behaved differently in RE4 in different areas where in RE5 we saw very little of that. The AI felt a little more advance on the Game Cube version of RE4 imho, but the AI did feel more aggressive in RE5. They ran after you more rather than slow down as they got closer to you. The enemies of RE4 were in my opinion very creative and well designed. RE5 had some fantastic enemies as well. And the Wesker fight's were well designed minus the final battle, which I think was poorly programmed as he could flick you into the lava without even hitting you.
In the end I love both games almost equally for different reasons