Why is this a problem exactly? Military strategy can be divided into two main functions: Offence and Defence. Before the British Empire forces were introduced; only the Offence aspect of war was present in CoH2. Now, we are able to have a Defensive play style too.
Some could argue that games have been too fast paced; far far too fast paced, usually the fault of gamebreaking OKW.
We have a faction called Ostheer that's supposed to be pure defensive. Yet they suck at both defense as well as offense. Look at their defenses and then the Brit ones LUL
They trump Ostheer in everything.
'Gamebreaking OKW'. That 'Gamebreaking OKW' had a really low winrate for most of it's life.
One of the biggest mistakes was to give UKF brace. It's simply illogical. It's like giving the Tiger a minesweeper upgrade and the ability to fire Falls/Ober smoke rounds at AT Guns(looking at you there Comet).
Another alternative is to make Bofors and Mortar pit exclusive from each other. Alone the Bofors or Mortar pit aren't really that dangerous, it's just that in combination with each other (and an AT gun) they counter absolutely everything.
And of course the cost. 400mp for a mortar pit that needs over 1k mp to even challenge, and that can withstand direct hits from Sturmtiger and Railway arty.
Compare the cost/effectiveness to OKW Flak HQ and OKW Flak Bunkers