I have concluded that MECS are useless. They might look pretty cool, but they have severe issues.
1) MECS require a ton of Meld to build and especially upgrade. I learned this the hard way when I decided to deploy three MECS. I feel the Meld is much better spent on genetic enhancements.
2) They have even higher aim penalties than Heavies do. I mean seriously, I feel like they can't hit a barn door unless they are standing right next to it.
3) Despite being mostly machine now, they are still subject to mind control unlike SHIVs.
4) They gain experience slower.
Meld is much better spent on genetic enhancements, especially on Psi troops. Psi assault and support troops with neural damping, hyper-reactive pupils, adrenal neurosympathy, bioelectric skin and adaptive bone marrow are absolute terminators. Equip them with ghost armor, alloy cannons and plasma rifles and end-game becomes laughably easy. Heavies and snipers are a lot less useful. Snipers do have convenient squad sight, but it is way too situational and it's really up to luck how useful they will be in a mission where map is random. Archangel armor negates this somewhat, but I think I'd rather give the slot(s) to either assault or support instead. Heavies used to be easier to pick with their +100 HEAT bonus, but now that it is down to 50% in the expansion, why don't I just pick assault or support instead? Assaults deal just as much damage and support can take over suppression role from heavies and still be able to heal.
4 assaults and 2 supports is my preferred go-to squad. High mobility, high utility, high versatility and doesn't lose in sustained firepower. A MEC that can hit with its particle cannon sure deals nice burst damage, but when it misses half the time...