Midnight-
You never see pings from other people's hits, you just see the tracer and any damage caused. So unless there's visible damage like missing parts and smoke, it's hard to tell whether or not anybody else is getting hits.
However, I do agree that acks don't seem to be able to hit stationary ground targets. This is just from looking at the tracer and the ground strikes--they always seem off-target by 10 yards or so. I've been killed in GVs by acks, but that was only when I was moving, like approaching an nme field.
Furthermore, I've only been killed by acks when I was driving an M3 or M8. Never in a Pz4 or FP. The Pz4 I can maybe see if you assume the 37mm is shooting only HE. Maybe you'd lose a track or something but there shouldn't be armor penetration. However, the turret of the FP isn't any thicker than a halftrack so you'd think acks could kill them fairly often, considering the turret is nearly as large as the hull.
The other day I was manning a 37mm ack at some field and spotted a stationary nme FP 1200 yards away. I quickly found the range with single shots and then held the trigger down. I hit him about 50 times and apparently caused no damage at all. I only stopped shooting when he, having noticed where all my tracer was coming from, knocked me out. He was still there when I looked from the tower after that.
It was my understanding that there is some randomizer in automatic weapons so that they don't always shoot in exactly the same spot. Instead, the bullets scatter out a bit around the point of aim to simulate the gun vibrating and all. At 1200 yards, therefore, I should have been hitting all over this FP, and about 1/2 of these hits should have been on the turret, due to its size compared to the hull. So at the very least you'd think I'd have shot his gun off. But I didn't, and I can't blame lag because this went on so long and he was still there after it all.