It’s incredibly useful, allowing you to KO an opponent from distance with no body found, and one of the only tools in the game (maybe the only tool?) that allows you to KO an opponent, steal their clothes, and then have them woken up naked and returned to position with still no body found and no alarm set off.
The gun itself is near silent when firing, and the ability to tranq a guard and then throw a brick at another guard while the first is getting dizzy is a great way to get past checkpoints manned by two guards.
And its two bullets can be upped to three when combined with the Seiker, or even 5 on a map like NYC where there is a spare tranq gun.
And when it’s empty, it serves as a useful placed distraction to send guards on a walkabout.
If anything, as a tool, it is overpowered.