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.
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.
How can the OKW trucks be brought into this? Get an AT gun and shoot it from range. It can't barrage and counter it's counter. Also it costs 300MP and 135 fuel and you can only build 1.
You can't counter with a mortar if the person has a brain - they just use the barrage and kill the mortar - or they have a mortar pit of their own.
Brits get two howitzers for teching, USF gets free squads (one of which can call in recon and arty). OKW gets a base it CAN set up outside it's base sector although dangerous.