The M20 isn't immune to small arms, the bazooka requires exiting the vehicle, and the gun is basically worthless versus any other light vehicle, and not even all that great versus infantry. (222/luchs are obviously better, but the 221 pintle MG and WC51 truck MG are also better)
You are right - but 2 things:
1. Given the vehicle's price that is hell of utility for free as all these abilitities (whether you think they are good or not) come basically for free and that tips the balance.
2. You compare it to OKW and that is a good comparison and a rather balanced match up - the problem is with ostheer where 222 is much more voulnerable to small arms and lacks those abilities . The solution may be to give some of similar utilities to 222 (smoke, maybe a possiblility to cap territory or just make it immune to small arms) and then paradoxically USF will be fixed against ost.
The fact so many axis players bitch about the mine is hilarious considering how often the shoe is on the other foot with teller mines, and how rarely these mines are actually laid down in 1v1s considering the million other things USF needs to spend munitions on (including the armor(hp) upgrade you so mention)
It's no axis platyers' bitching. I'd say it is allies bitching about tellers not really realising they have their own tellers. Good players lie them just like axis tellers and they can be deadly as axis lack crit repairs (with the exception of tiger ace).
Each factions requires munitions and trust me axis need them too, for ost you must often decide between a teller, healing, infantry upgrades, sweepers etc. and usually for me it is harder than when I'm playing USF.
Basically, the M20 is overboard with utility because it's literally called a utility car and it's not particularly great at anything else, even chasing down snipers after the USF tech patch lowered axed its near main gun accuracy. That said, ditching the bazooka would probably be fine since 9 times out of 10 its a death sentence to try and fend off a 222 with them anyway, and ends with a destroyed or stolen M20 instead of a dead 222.
Well, not really true as many players replace crew with rear echelons and have a free bazooka squad early, which has huge impact on 222 and halftacks early game. Again it is more overboard than you seem to realise. Also the ability to absorb a faust shot and quickly come back with crew repairs can drain your opponents of munitions without giving them any breathing room.