What am I missing about Metathermics? I often find myself in positions where I'm faced with the choice to spend PSI points on a Metathermics ability or something else and I often wind up getting more mileage out of the other ability. To be honest when going pure PSI I feel like I have TOO many options.
Metathermics is probably going to be your most efficient bread-and-butter type damage against most organic targets. You're really going to want Pyromaniac and Thermodynamicity for it, though. Pyromaniac is going to give you group crowd-control via fear in that pre-level 14 zone where you can't just winbutton LoC+Enrage and end turn until it's time to loot. It'll also give you a devastating opener in the form of Psionic Mania + Cryo Orb if you've gone Psychosis; that opener leaves bleed debuffs which won't disappear when you follow up the Cryo Orb with a Pyrokinesis, overwriting Chill (a weak debuff) with Burning (probably the strongest debuff in the game if the target also fears burning). Cryokinesis is a cheap, very-long-range puller that also inflicts Chill which means it's your ideal kiting ability against heavy-damage melee types, or an excellent addition to psi slow when facing enemies with out-of-control high move points. The fire avatar ability will both let you cheese stealth melee enemies (Crawlers especially, which do not willingly run into the flames while stealthed and instead wait outside them long enough for you to find them and kill them at no risk to yourself) and also generally disregard fire in all forms, meaning you can safely molotov yourself and also trivially cheese Industrial Bots. If you just hate the idea of challenge at all, then you can just beat almost every fight in the entire game (only four come to mind in the entire game that are highly resistant to this) by just making some noise, manually starting combat, hiding behind a four-turn force field, then popping out and triggering a ThermoD explosion to destroy entire groups in an instant.
If you don't want to cheese ThermoD, and are playing on DOMINATING where you can't always (but can usually) wipe the entire map with just a cryo orb + fireball combo, then Thermodynamicity will let you output horrifying amounts of damage by alternating hot and cold. This isn't a whole lot of help early on because psi costs are your limiting factor, but later on in the game it's action points that restrict you, not psi points, so being able to dump eight or ten abilities in a turn will be a huge help.
Now, of course, you don't need to do any of that. I'm not trying to convince you to
like Metathermics, or even use it at all. I just want to point out that if you feel like it's weak, that's because you're not using it well, yet.