The gene squad is pretty powerful, but the mecs have serious usage as well. Believe it or not (seriously) I built a balanced team for ****s and giggles (we're talking 1 heavy, 1 sniper, 1 assault, 1 support, 1 mec and 1 shiv). It actually works REALLY well. I gene modded the hell out of the normal classes though, and made sure I got to hover shiv asap, and the mec trooper was a support base starting out (I kept him back for sneaky flaming behind corners). Particle Cannon was ok I guess, but it was just for extra damage.
I think I built him flamethrower, grenade launcher, mine launcher. The grenade launcher isn't powerful, but it was not bad for helping destroying cover AND hitting mind controlled targets (and it's range is pretty decent). The mine launcher I found out is best used NOT for stopping things from flanking you, but for setting up a combo kill for whoever is next in line to shoot. Drop a mine in the middle of targets, let someone else fire, and it sets off the mine usually. If it's a berserker, they move, setting off the mine anyway and hurting or killing their friends (8 damage from the mine plus I think 15+ from a crit sniper shot softened up beforehand with a shredder rocket? Yes please).
Mimetic skin is good for most troops, but the electro version (the opposite I guess) is good for assaults due to seekers I've noticed. My little sentry I run to the corner of a building has more kills than... hell, I dunno, she's got over 100 just from sitting around and waiting for stuff to come to her. The rest of the squad wishes they were equal.
From what I've seen, it's not really what is more powerful or which does more damage, it's what works best with everything else you're using. The hover shiv is my scout and my meld grabber, and it works EXTREMELY well doing that. The mec is best as support, not as a real tank (which means I could actually just swap the nader for a heal mist, but eh).
They never designed the new classes and abilities to be better than other things, they wanted them to work in different circumstances. Regardless of what anyone says, what works in one battle may not work in another, or it will just depend on your play style.