The units I listed as situational are all useful but the frequency where I actually needed them to win is low.
Engineer: aside from playing maps with pre-placed capturable buildings, I only use them to recapture buildings that are captured (if the enemy didn't sell them) or if my construction yard and WF/Airstrip is gone. Sometimes I engineer rush in the beginning, but that's like once in 30 games against the same player so he doesn't expect it from me. Best to do it when they are caught off guard.
Commando: Very useful if the map has run out of tiberium. People will sell buildings and have lots of infantry, ability to easily demolish a building is very good because losing a building in this situation hurts you a lot more.
Flame/Light Tank/SSM/Artillery: I find them very useful (even better than the Tier A units) if the enemy doesn't have medium tanks or recon bikes, usually when they lose their cons yard and WF/Airstrip and have lots of infantry (I use the rocket launcher for this situation too). I also use them against people camping behind lots of AGTs. I don't usually build them outside of these situations because I go bikes and buggies all the way.
Transport units: Only useful with commandos and engineers, which are both situational units.
My group A units are the units that I build 99% of the game and I can rely only on them on many 1v1 games.
As for support, I can win many games without them but they make the game easier like if my opponent has lots of rocket soldiers, I can still win using only medium tanks but I bring along some MLRS to destroy them quicker, take less medium tank casualties, make it less frustrating,etc. There are times where this group of units saved my game (like stealth tanks and helicopters help take out important buildings) but that doesn't happen much , usually on games with many players.