Vet5 STG volks are good for late game, they can stand more or less ground vs vetted uped inf somewhat, problem is all their power comes from vet and if you lose vetted volks there is pretty much no replacement aside from obersts\doc inf, since its already takes almost full game to vet them up.
Other mainline inf at least have powerfull upgrades (like 7 men cons) or weapons upgrades to make even un-vetted new squad somewhat usefull.
For combat stats only, volks with max veterency get
-24% recieved accuracy
+30% weapon accuracy
-20% weapon cooldown
For comparison rifles get
-38% recieved accuracy
+30% accuracy
-20% weapon cooldown
Ostruppen get
-28.5% recieved accuracy
+40% accuracy
-20% weapon cooldown
Conscripts get
-37.3% recieved accuracy
+50% accuracy
-20% weapon cooldown
The only mainline infantry that gets worse combat bonuses is the infantry sections who get less recieved accuracy reduction and less accuracy-but make up for it by having the lowest stock recieved accuracy of any mainline in the game by a fair margin.
My point is that volks vet really isn't that good. The passive healing is great and imo it wouldn't hurt to put it back to vet 3 but other then that, they just don't scale as well as pretty much any other mainline.
I am not saying they should be buffed, but they are definitely on the weak end of mainlines.