However, I imagine it would be difficult to control collusion in games like this, which is probably why there are so few around.
Yeah, the first Bitcoin PvP I tried and stuck to for a very long time was precisely poker, on Bit-Exo back when they had a lot of players. Collusion was definitely an issue but not a big one there, where you had a small community of everyone who more or less knew (and somewhat trusted) each other. So you knew yeah, those 2 players would always gang up on everyone else. Fair enough, alliances were more or less transparent. But I can see how it'd be difficult to manage online without some form of verification (that we Bitcoin gamblers stay away from!).
What's really annoying, more so than that, imo, is bot players. I hate those with a passion.