Pak40 is slowest reposition, has got the most counters, has lowest vet bonus and has to supplement wehr weak doc armor against TD.
Its not, the Zis gun is by far the slowest at gun re-positioning wise. Im also pretty sure the pak 40 and 6 pdr are stats wise basically the same. The only AT gun that outperforms the Pak40 is the USF 57mm.
Also any at gun is countered if you can't use it properly. Also not sure how it has the most counters, every faction in the game has nondoc units that counter static at guns or positions.