I play chaos knights, I run 8 models in my army.
I find they are fun to play, but the games are basically decided turn 1.
If my opponent can kill 1 or 2 of my knights or armigers turn 1, I will probably loose, if they can't, I usually win.
Anyone with transhuman (dark angels for example) are really hard, as needing to wound on 4's sucks, and folks with lots of inv saves are tough too.
I find playing the mission isn't too hard especially as wardogs are obsec.
I run a tyrant, desacrator, and 6 wardogs in my army, with 85 Reinforcement Points for summoning demons.
I've played vs grey knights, necrons, horde and elite tyranids, space marines with my knights, and only the dark angels really caused me a lot of trouble.
Haven't played against any of the meta boogeyman armies, mostly due to me not having opportunities to do so.