Just tested it.
P4 does get penetrated but slowly. The P4 still wins every time with a little over half health.
Panther on the other hand is almost completely invulnerable and the bofors could only take off a sliver of health before dying.
The flak HQ however was able to kill the t-34, sherman, and cromwells about 50% of the time give or take, so the OKW buildings are superior when it comes to AT.
In AI too given it has further range, pinning, and a ton more health. so the only advantage the bofors has over the flak HQ is it's barrage and brace abilities.
See this?
Do you guys even play the game before you start crying something is overpowered? If emplacements are so OP, I would gladly take a mobile ISG than an overpriced stationary mortar pit any day. Brits are in need of a buff, not a nerf. ISG spam + flak truck is borderline impossible to deal with as the British, you have shrecks which completely annihilate any sort of early armor, (which is now replaced with faust ability which is still just as bad) and OKW even has a non-doctrinal Stuka which completely annihilates fortifications.
OKW has the upper hand in pretty much every stage of the game against UKF.
Early game : Sturmpioneers are a T0 unit that completely destroy any infantry, they're stronger than royal engineers which come at T2 for Brits. They fire on the move so it's not as easy trying to fight them from range (they shoot at you while you try to run away, especially when there's a volks with the sturmpio)
Early-mid game : Even if Brits somehow push out OKW despite OKW's far superior early infantry, setting up a Bofors is difficult, engineers take exponentially more damage when building, plus ISGs hit the field the same time a Bofors can be built. By the time a Bofors is finished, you'd have ISG's already bombarding it before the British can even build a mortar emplacement, and by the time a mortar emplacement is finished it'd be already too late.
Mid game : Brits lack any sort of reliable AT, OKW have shrecks which can destroy the heaviest of Allied vehicles while Brits have PIATs, and no AT grenade. What does this mean? Flame half tracks practically run rampant, the lack of AT grenades means you can't disable vehicles like the point and click panzerfaust ability does. Literally all you have to do is move while burning everything to the ground, the PIAT's will never hit.
Late game : Axis heavy tanks. Enough said. Oh, you somehow managed to survive to the late game? Here, have Stuka barrages + ISGs for your emplacements while I sit back with my King Tiger and shreck blobs.
The OKW has multiple opportunities to get back into the game, even if they lose a position. They have ISGs and flame track timings which they can use to destroy British emplacements (bombard with ISG first, when Bofors braces go in with flame track). Do you know what British has when the OKW sets up a flak truck at a strategic point? Nothing. They could build a stationary mortar emplacement for 400mp at a position for the sole purpose of attempting to destroy the flak truck (and will be completely useless afterward even if it manages to destroy the flak truck despite ISGs), or they could build an AT gun which is cannon-fodder for ISGs. After the British loses a strategic position it's GG, they have no way of getting back into the game, and even if they do somehow manage to destroy a dug in OKW they will be too far behind in fuel to get back into the game. Axis tanks will come and annihilate everything.