Here are some units that I personally underuse. I try to be as versatile as possible and use every aspect of the arsenal, but there are still some powers/units that I neglect.
Siege Cadre - Underused. Cadre don't seem very versatile, they're only useful against buildings. I'd rather build a lot of light tanks, which deal decent damage against structures and can withstand an enemy attack. I wished that they could deal good damage against light vehicles because Allies seem to lack a strong T1+ anti-armour infantry.
Sonar Pulse - Naval is just plain boring. Never used.
Bomb Buggy / Demo Truck - Suicide units just seem unreliable to me, I underuse them. I prefer Borillos and Armadillos because they can actually contribute during fights. I once read that people prefer high-risk, high-reward even if there's a more safer option. This might be why I love spamming MADMAN rather than bomb buggies / demo trucks.
Vultures - Catastrophes are already extremely versatile, so anti-air isn't a problem when I play LC. I only build 2-3 vultures to use smoke-cannons. I don't know if this counts as underused.
Aerodome - Since Aerodome is entirely optional for the Epsilon tech tree, I tend to skip building this. The jets aren't that great either.
Grinder - I forgot this existed. I'd rather send mind-controlled units to weaken an enemy attack anyways.
Signal Jammer - I don't know how to use this power effectively. You need to predict when enemies use their support powers, but it's quite hard, so I usually forget this exists. It might be more useful to cast it on your army as a protective shield. Against smart players who actually kill Raccoons, signal jammer may be useful because it can't be shut down by killing a unit.
SODAR Array - With Last Bastion relying so much on SODAR, I feel like Mentalmeisters go mental whenever people don't use SODAR. I typically build one, plant it near my Sweeper/Giantsbane/Shrike Nest defenses, and forget that it even exists. SODAR is a bit of boring mechanic. You deploy it, attack harder.
Duplicants - Godsbane/Zorbfloaters are too powerful to waste Nanofiber Sync on a clairvoyant. Huntresses are the same tier as Clairvoyants, can throw nanites in range, can hijacks, and don't need to be sync'd. Huntresses make duplicants pointless. Duplicants should be repurposed into something else entirely.
Diverbee - Similar to the Duplicant. Megalodons are already extremely powerful, so there's no role left for Diverbees to fill. Maybe Diverbees need to be cheaper.
Blast Furnace - Giantsbane with SODAR is more versatile. Alanqa Skystation seems more potent. Haihead just spam Teratorn + SODAR. The only reason why I would ever use this is against Salamanders, because they're too strong to handle with any Foehn unit.
Shadray Sonic Tank - Syncronin + Megalodon seems to be reasonably effective against structures, infantry, and units. I neglect using Shadray even for anti-air. I prefer Teratorn spam.
EDIT: Nvm, duplicants are quite strong. They turn infantry spam into an infantry farm. This is definitely going to shift to overused.
Edited by aethiraes, 28 January 2017 - 02:57 PM.