Nightreign only supports 1 or 3 player modes, the mod lets you play with just two players.
As a somewhat busy adult with work and kids, finding time to play together with friends is kinda hard. We have some set times a week where we try to play together (but we have more than 3 players) and we often squeeze in 2 player game sessions when possible. Nightreign only supporting exactly 3 players for multiplayer didn't really work for us.
Some games don't get any performance increase, so you'll have to try it game by game. So far most games I've tried have worked, but maybe I've just been lucky.