We went through a game with Doom Waves on but we set the whole difficulty too low so we just spawnkilled them. Next time we'll try setting 5 waves with +1 Doom each wave, that should be a good start.
What worries me is that each Doom unit is basically a walking HP pool with long range, maybe you should give each of them their own gimmick (Colossus pulling units fast with its arms, Fatboy periodically spawning group of enemies, (if that's even possible) for example). Also, as usual, an option to set their respawn time (or remove it completely) could be nice !
We haven't tried with endless waves and bosses either so maybe they work best with these on ! In any case I like the idea and this will make the endgame much more fun