Ok so let me explain the basics. Since you don´t seem to be able to understand what both vehicles are used for.
OKW AA HT -> needs to be stationary but if it is it suppresses infantry way better than USF AA HT and after vet 2 becomes a killing machine. Largely useless against vehicles. Can do very well against Soviet T1 or USF LT because handheld AT-options don´t work well against it. Rapes planes at any stage of the game. It´s supposed to act like a bofors on wheels.
USF AA HT -> very fragile because it lacks smoke, can suppress on the move but takes quite long to do so, doesn´t deal as much damage/suppression to infantry as OKW AA HT, does well vs 222s if microed well. Harder to use vs planes because it has no 360° gun rotation, some people also claim it´s bugged and doesn´t shoot at planes etc. It´s more of a traditional LV and can be mobile.
Which one is better or worse is a matter of situation and personal preferences but I personally hate the USF AA HT because it is so easily countered by cloaked raks or a Pak 40.
Neither USF AA HT nor OKW AA HT are meta in 2v2. Stuart or Luchs/Puma are the prefered options. And there is a reason for that. So no the USF AA HT is not OP.
I am already quite aware of their differences by know, having played this game quite a while.
Look to this comment
https://www.coh2.org/topic/90446/usf-okw-aa-vehicle/post/746076
I may not have stated every point but more than enough to know what they are both viable at.
You are quite right about vulnerability wise but the thing that in some way compensates the USF AA is due to the fact it can be mobile. Sure you would like to set up to make the utmost use. Usually a good player keeps it mobile when alone without support to harass. When he has the support, you can use it more comfortably.
FHT problem is its AOE for little damage overall. It is 1 and I think it should be increased by 2.
Comparing its performance against infantry. USF AA tends to be more conistent in damage, kills and surpression although it does suffer from AA tracking which is not a big deal compared to more important just as the pros I just mentioned. It can be mobile, and does something on the move. Have crew to repair to rengage faster. If you know you are going to lose the vehicle. Decrew and you wont suffer entirely. The only real downside is no smoke.
The only I good thing about FHT is smoke and AA tracking capabilities. It is not that lethal against infantry. The reload is slower in comparison giving it huge breaks causing the surpression to no longer last and the damage is just puny. Downsides, setup, vet speed, damage, AOE and reload.
16 damage is just lol. I am fine with 16 but the AOE is another joke since it is 1 while USF AA is 2. How does USF halftrack have nearly twice the consistency and damage. Better reload, AOE is 2. Just wow, so astounding the difference. Shoot on the move and does not require a unit to repair since it has crew. Can counter Luchs easily. .
Currently OKW build choice only is Luchs for proper AI. That is just a terrible mechanic. Either buff FHT or remove Infrared Halftrack for a different AI or AT unit.
Either AOE or Damage increase for FHT would definitely help!
Just play in game and see the differences in their performance. You might just reconsider!