Those 3 ISGs ( 27 popcap) or mortars might seem like a great idea until a couple of Cromwells appear from nowhere or vetted infantry finds them with Brens.
Think of emplacements in a resource orientated way. That bofors guarding your fuel and VP cost 30 and 280MP. It lasted maybe ten minutes? Bargain. for gawds sake let it die. You got 80 Fuel in those 10 minutes. Anvil engineers can repair under fire, engineers just get wrecked and barely postpone the death of the emplacement. How much manpower did it save you though? If then answer is more than 10 Tommys then profit.
Pits too, annoying some crappy pios or Volks early game is one thing, late game when infantry is thin on the ground because armour is taking up 50 popcap, and the pios are needed to repair, he has to use vetted death dealers to cap. Is it worth 400 MP to take out a single Vet 5 Ober? To weaken them enough that your infantry can take it out? Absolutely. Late game small arms do less, mortars though still wreck. A late game mortar pit is an excellent way to farm rage quits.

If the opp is mounting an armoured assault supported by IDF and offmaps to take out a piddly Bofors for gods sake just let it die. Repairing under fire is something only Anvil engineers are designed for, otherwise you are just bleeding yourself in a game you can't win. Not to say targeting his inf is unhelpful, just retreat before the big guns turn on them rather than the emplacement. If he is dedicating that much force to one place it should mean opportunities elsewhere so research grenades, buy a UC and take out that annoying MG42 or whatnot to take his cutoff / fuel while he is at it. You can then build a Bofors there and rinse repeat, probably in a better position where he doesn't have the depth to comfortably use ISGs etc. Don't think you have to stick to one side of the map.
Also don't forget the best emplacement of them all. The humble trench. Time is another resource and the number of times the opposition will stop to take out an empty trench is amazing. Course a sniper in one every now and again will keep him wary... The best thing about them is that some of the many counters against emplacements suck arse against them. Paks and tanks in particular. Also you can happily build them out of sight providing you stay away from a capping point.
All emplacements are a fixed cost, and none really give anything really useful in terms of vet. Don't get attached to them and stay mobile.