And OKW trucks have more range to cover a larger area, more health, AND pinning to snare any infantry that comes into range. On top of this the schwerer is just a tech building meaning you still have an full army to protect it. While the bofors costs both teching and the cost of the unit, meaning you have less units to actually assault besieging units with.
AT guns are the counters to most other emplacements and buildings. I know this because thats what everyone uses to kill my okw trucks.
The bofors needs to contribute something to it's defense, maybe that doesn't need to be outright countering the counters themselves, but otherwise all the all the german player needs to do is simply buy a single cheap 240mp mortar squad and defend it and the brit player has to somehow beat it with a much lower unit count. The bofors will pretty much die every time and then there goes the brit players entire midgame.