From my gameplay experience, this is my list. I'll only do support powers for now.
Sonar Pulse - Underused. Not really much to detect submerged units in a small radius when Dolphins, SEALs, and Destroyers can do a much better job at detecting and countering. If it functioned like it's RA1 counterpart, where it revealed all submerged units for a good five seconds, then it'll have more use.
Backwarp - Overused. Gives free healing for any Allied unit blob. Makes Medics unused since it heals infantry faster, and especially makes an Allied naval fleet last longer, ensuring their naval supremacy.
Mercury Strike - Overused. Gives USA potshots at infantry and vehicle groups, and more damaging with Target Painter.
Terror & Tank Drop - Overused. Nothing more annoying when playing against Russia, as they can spam their Rhinos and Terror Drones to make painful ambushes (more so with the Iron Curtain, which makes dealing with them a pain when playing as any Foehn subfactions.)
Flame Tower - Underused. Pretty situational defense, and that the best use I can get out of is guarding a tech structure from enemy engineers. You'll get a lot better use from using Instant Shelter, and it's available earlier once you built a Field Bureau. I also wish the Flame Tower replaced the Sentry Gun for all Soviet subfactions, mostly because it looks cooler.
Instant Shelter - Overused. Gives the Soviet player a powerful defense anywhere on the map.
Vision - Underused. Foreseeing enemy movement is useful, but it's just that. Decent scouting and radar/map awareness makes this obsolete.
Bloatick Trap - Overused. Removes infantry and/or heroes without retaliation. Also highly damaging on fragile structures (Power providers are especially notable.)
Toxic Strike - Not used. A useless waste of $1000 that only annoys infantry and does nothing against anything else.
Devourer - Overused. Like Bloatick Trap and Mercury Strike, it sweeps infantry effectively.
Megaarena - Overused. Gives Megalodons the best damage and armor upgrade in a wide radius, with the added effect of vaporizing any infantry nearby.
Signal Jammer - Underused. Helpful for a short period, but spamming Raccoons do better ay preventing use of support powers and superweapons.
That's all for now. Might add more in another post.
Sonar Pulse - its a scouting tool, so what? Plop it on an enemy Naval yard and have a look.
Backwarp - You're describing a support power doing its job and then saying its overused as a result. Most Allied ground armies (with exception to something like a BattleTort Spam) need Backwarp support to stand against the much heavier stuff that Foehn and Soviets can bring.
As for Backwarp making Allied navy too powerful in lategame...
Also Backwarp requires a Warpshop *and* a Battle Lab. Its a super lategame ability, of course its fucking useful.
Mercury Strike - Again you're berating something for doing its exact purpose. It supplement's USA's lighter unit compositions as well as buffs Athenas if you're in a really tricky siege scenario.
Terror/Tank Drop - Ok these support powers are intended to supplement your army to boost its numbers, usually people just drop them in very random places to get free sneak damage, and often in the case of Tank Drop, just get a freebie Techsnipe or SW snipe (because thats 100 times more powerful than having extra damage zzz) Frankly I don't like this at all but it happens.
Flame Tower - Everybody says that "oh look flame tower is useless etc". Sure its a bit *niche* but its not entirely useless, and costs next to nothing to even use the damn thing anyway. Heck I usually use it to support Arsonists by burning engineers with them.
Instant Shelter - Overused? I have almost never seen someone use a strategy revolving around Instant Shelter, like ever. Literally never. Once a player hits T3 that Bunker gets deleted and nobody uses it ever again. Sure it has *some* viability as a tactic but why go for T2 push when you can just make your Hero/Epic Unit/Fuckton of Desos and go win that way?
Vision - I *kinda* agree. Its more underused because people forget it exists, but its also crazy useful in Epsi vs Epsi, for obvious reasons.
Bloatick Trap - I mean, if you're just going to sit there and not pay attention to your dudes, of course I'm going to use it on you. Most heroes are far too tanky to die from this anyway, and in the same bracket as Mercury Strike its just doing its job.
Toxic Strike - Ok I 100% agree on this one. Shouldn't need explanation.
Devourer - See Bloatick Trap. Also is one of the few ways for Foehn to heal their dudes, AND it needs reprocessor, which is pretty lategame stuff.
Megaarena - Back in the day I'd agree with you, because lolxd Megalodons in Megaarena was so stupid powerful people forgot that Haihead could build other units. Since then not so much, its been nerfed multiple times and now people just crutch on FinAlize and MADMAN rush anyway so who needs em?
Signal Jammer - you mean one of the most annoying cancer structures in long games? That you know, can make Foehn un-siegable? (No I don't mean Blasticade lol) It costs 2500 and a shitload of power for a reason.