The only thing I saw consistently being shot down are the supply drops from Land Lease. IL-2s and P-47s seem to be less vulnerable to the FlakHQ, they still deliver most of their payload while under fire from it. The strafing runs from Tactical Support and such are directional, so just avoid the Flak.
Don't forget Recon Plane, Lelic when they design USF initially they thought it cool that only US got high-attitude recon not counting SU, with Flak HQ SU can say goodbye to air drops, not to mention Soviet got the weakest AA to boot. Generally, all Allied options for AA sucks, Brit got Bofor/Centaur which nobody uses unless you go cancer spam. USF got AAHT which is expensive as fuck, better use Sherman top MG. Both OH and OKW got nice AA compared to Allied. (222 and Schwerer, Pintle MG)