Both vehicles can be damaged by small arms, even after the armor addon for the M20 (which is primarily a health bonus iirc). Armor value has to be over 35 to ignore most small arms. Overall the M20 is more resilient with or without the upgrade, thanks to crew and smoke, but it is also less effective versus infantry than a 222 and basically helpless versus other light vehicles. IIRC even the 221 will outdps the M20, though the latter unit scales better with veterancy.
Primary AT/Anti Light Vehicle capability of the M20 is always going to be the mines, which are as dicy a proposition as any expensive mine in this game considering a detector squad can erase that investment without any impact.
Anyway, bazooka nerf is fine. LT gets a zook now so no big deal.
Also, the vet 3 penetration boost for the M20 is absolute garbage so I don't want anyone bringing that up. At most it adds something like 1.5 penetration at minimum range, bringing it to a whopping maximum total of 4.5 and a minimum of 1.5 at long range. If it were my call I would have made it a flat +2 or +3 penetration bonus, which raises the penetration stats up to normal vehicle M2HB levels.
Armor skirt is hardly "primarily a health bonus". It increase the EHP vs small arms of the unit dramatilly.
As for the chance to be penetrate by small arm fire once upgraded, yes it can be penetrated but the chance is rather low thus grealty decreasing repair times.
"helpless vs other light vehicles" is rather misleading since it can beat the kubel and 221/223 which are the other "light vehicles", unless you want to talk about light tanks.
As for vet bonuses it gets an accuracy buff at each vet level so there is problem there.
The unit is extremly cost efficient and can survive and be useful even at late game.