Emplacements are probably the hardest thing in the game to balance and at the moment it just so happens they are way underwhelming. It sucks for the British since a lot of their design is about playing around emplacements but I honestly prefer them being somewhat weak tbh, it makes for more interesting games
There are good threads and posts on how to balance emplacements. These are the suggestions that I like:
-Buff HP
-Tweak Brace by using target tables: Light artillery pieces (mortars, ISGs) should do minimal damage to Emplacements, even without brace (something like 50% damage reduction) and even more when using brace. Heavy artillery pieces should do decent damage, even when braced (100% without brace and about 50% with brace).
-Small arms should be able to damage mortar pits when not braced. Getting close to mortar pits should be encouraged.
-ATGs should do decent damage to Bofors and Mortar pits even when braced (50-60%) but not 17pndr.
-Nades should do decent damage even when braced.
-Incendiary weapons should do decent damage when NOT braced, but reduced damage when braced.
-Brace could be a Toggle ability with small cool down.
Still, as you said Emplacements are kinda hard to balance, but I think some of these suggestion (Which I've gathered from different posts) might be a step forward. Needs testing though.