Trygon etc.

The Dimachareon. It basically is the CC beatstick that the Trygon used to be, except better. Although I can't recall if it's Fleet or not, a must-have for any combat-oriented unit (at least on foot).

Mawlocs are cheap, and can do some real damage, but suffer from "shooting" every other turn, and being generally inaccurate. It's probably easier to justify them in most standard lists, though, and you can build around them (lictor homing beacons) to some effect.
The Trygons give you safe DS on a decent combat build, but little else. The Tunnel they leave is, 9 times out of 10, a complete joke, unfortunately, as it would have opened up insane potential for all sorts of things had it been altered to something generally workable. A brilliant idea wasted.
Primes are the above plus Synapse, and a better gun that you'll only shoot once anyway (but in fairness, 12 S5 shots does threaten AV 10 something decent).

In short, I haven't seen these guys used too much, outside of niche lists to make their quirks more prominent (Tunnel works well with the Endless Swarm formation, for example). Generally speaking you want something that can shoot, and while the Mawloc sports infinite range with a pretty solid "gun", its lack of accuracy makes it a crapshoot at best.