Indeed the 20mm quad flak is one of the best cannons in the game! I destroyed Super Pershing Jumbo and all allied stuff in CMBO and CMAK where I employed it.(I didn't used it in CMBB so I'm glad it's the same old good gun)
Actually the 20mm quad saved me a few times bringing even a victory [what a victory] ;) and it's advantage it's stealthy
The best use for them immobilize or gun damage enemy tanks and they will abandon the tank pretty quickly.
I was unhappy one time when facing against a TD(Jackson I think) although I fired numerous times even from SIDE and REAR and from a distance of bellow 100m where I had the odds for me I couldn't get the side/rear turret/ hull penetration knock out :( but destroyed the tank the second salvo after track hit immobilized )
My question is if this gun it's actually a Heavy MG with timed explosive projectiles why doesn't it wracks havoc against infantry :( yes they pin down the infantry...rarely getting some kills (for low calibers)
The same question stand for all flak cannons especially the 88flak which strategy in Africa was to fire timed projectiles so they would explode above the dune killing enemy soldiers not on ground