Hm...are you asking for underused or useless? The titel says underused, but Dusters are useless. If we're talking about useless units, the G-Boat is definitely one of them. I would have to say that Artilleries are underused.

Dusters are usually the best units to put a CO into. It has High movement, It's Cheap, And Can Either run away or really hurt it's only threat(fighter). The Anti-Air can't get close enough to attack it.

I always put my CO in a Duster, Penny and Mr.Bear love the view.

A duster with Tabitha in it can take out 2 Fighters easily.

T-Copters I use all the time, they're especially good at buying an infantry some time to secure a city while the enemy is shooting it down. B-copters are also really useful considering they have 2 attacks, vs. tanks and vs. infantry.

Most underused unit for me is the carrier. Its only good at making Sea Planes, which cost 15000, but it already costs 28000, not to mention absolutely no defense or offense options. Sea Planes aren't bad, but the fact you always need an rig by them is annoying. One reason I would really love B.Boats in this game.

dusters are great for killing infantry, copters, and harming other aircraft. They also make this cool pingin sound when they attack tanks. They also make it harder to your opponent to get air superiority if they have the upper hand with funds.

Carriers are awesome, I dont care what anyone says. In maps with no airports, you can still make air units! They were not designed to go far beyond the carrier. Also they do well vs aircraft... yeah.

Gun boats are great. They are cheap naval units good for harassing other naval units, and they can be used to form barriers to protect other naval units. a few gunboats can prevent a bomber from making an easy kill on your carrier or battleship. they also make good patrol craft to search for subs. Personally I would rather a gunboat find a sub than my battleship or carrier.

B Copters. you already know what thats about. owning ground targets and blocking units.

Artillery can be place directly behind the front line and attack, rockets have to be 2 spaces back. you might say that anti tanks are better, and they are, thats why they cost twice as much. Artillery is more economical. on a small money map anti tanks cost is prohibitive for what you get.

While I never said that I don't use Carriers and think they're terrible, you'd have to agree that, due to their cost, it'd be pretty stupid to buy more then one. Which gets the most underused unit for me, since I can see uses for more then 2 of just about every unit but the Carrier. Flare is questionable, but due to its low cost, its nice to have an extra in FoW battles.

Carriers AA attack is pretty bad. Not to mention you really don't want to send a 28000 G ship to attack an air unit. You never know where a submarine may be, especially now considering that they are built sunk (which is awesome).

Well protected, they're the greatest thing ever. However, its open to attack by just about every air unit, and every sea unit. Considering its low defense, it just doesn't seem worth it most of the time.

too me, the carrier is the 5th best thing in adv war. if possible, with enough funds, i will do a rig+carrier combo. first, turn the rig into a port move the carrier in to the port after completion produce the planes. if you wish to move, make another rig and make it into a port again.