Based on personnal PvE experience, as well as looking at PvP matches published on Youtube:
--- Neverused ---
Skyray: Patriots feel more cost-efficient,
Battleship: never seen anyone using it.
Vulture: If I need to scorch an area, I'll rather use Buratinos. For AA, Catastrophes with possibly a flak trooper inside.
Hijacker: Why use this guy when I can use Epsilon adept that does the same thing from a distance ?
Blasticade: I don't know any scenario in which I'd say "damn I need blasticade". This thing is just so useless compared to other support SWs. It would fit if it was like Chinese EMP station.
Turmoil grid: Everyone seems to prefer stun grids.
Nanocoat regulator: probably because it has a 2*2 footprint and narrow range, it is harder to put it between buildings (that is the issue I have with it tbh)
--- Underused ---
Naval transport: Aside one player transporting his MCV to avoid being killed, never saw those in action
T3 heavy defenses: probably because of their high energy cost. Only exception is the plasmerizer.
Warp miners: very few people seem to use them. I think I saw some players using them as a diversion for an offensive chronoshift, but that's it.
Sonar pulse: I use it when I see the AI using pirhanas, so I can bombard them at long range, or when I am assaulting a seemingly empty base. Never seen it used in PvP.
Snipers: very rare sight. I tend to use them with IFVs, just to clear infantry.
Hummingbird: I've seen it a few times, mostly in long games where the opponent was spamming units.
Mortar quad/Motor ambush: Slower and squishier Speeder bikes, available at same tier, not reliable enough
Yunru: Only used in the centurion as long range disabler. Earthbreaker is so unloved
Stalin's Fist: Fun to use in PvE, but for some reason not liked in PvP.
Kirov: Other equivalents have an unique feature (radar-invisible Barracuda, omni-directional fire for Basilisk, crazy range for Quetzal). This one feels rather bland.
Mind reader: overshadowed by the chimera core. Why predicting your opponent's path when you can simply cloak your troops and ambush the attack ?
Bloatick: The built option only, the trap seems quite loved by SC players
Oxidizer: I like to use them in PvP, but again, never seen it used in PvP
Gehenna: The T3 AA with the longest range, but almost never seen in PvP. Too slow maybe ?
Syncronin: The perfect support unit on paper. In reality it costs too much to be useful, especially for a T3 unit.
Syncronaut: Losing his range for stealth and aoe. Works only if your opponent has a very compact base.
Angelshark: Too situational to be truely useful.
--- Overused ---
T3 "monster tanks": Whatever faction is played, those are often seen.
Desolator/Eradicator: very good vs units, a must-have, especially with the centurion
Chimera core: Probably will be less used after patch 3.3.2, but as of now, it is used in the majority of matches that involves epsilon
Signal inhibitor: I hate this thing. Put that in the middle of your base, and SWs are useless.
Harbinger: A second SW for Coronia. It is pay per shot, but still...
Quetzal: The most used of T3 aerial base killers. Contrary to Barracudas, they require minimal micromanagement. Plus, they can fire on the move. Often seen in larger groups than its equivalents.