Yep. You've pretty much got it! We know through Halo 4 and Spartan Ops that Prometheans have human memories, which one would think might make for interesting, conflicted enemies, but we were never shown anything resembling that. If only we had fought "real" Prometheans, elite Forerunner warriors still under the command of the Didact, or some such thing.

One argument I would make though is that English is not necessary for an enemy to feel intelligent or for it to seem to have personality. Halo 1's Elites, through their use of reversed speech, were still able to convey anger, fear, surprise, injury, and the giving of orders in a way that was understandable to players. All in all, it's very impressive how much Halo 1 elites are able to emote without using understandable words!

It's one of the strange pitfalls of the Halo series that Bungie got so much right the very first time. Since then that interaction between players and enemies has been very hard to build upon, much less duplicate into a new enemy race.