It is immobile, expensive and gets completely shut down by less expensive artillery units. Only situation when it is worth using is when you are completely sure your opponent is stupid enough not to build his own mortar.
I wouldn't see it gets completely shutdown by arty units. They're a counter, but they're like a bazooka up against a medium tank. Good-ish but hardly definitive. With the flame damage nerf the incendiary halftrack round isn't even as scary. The only thing that truly threatens it is an ISG, but iirc it needs vet to outrange it.
In general though if your opponent invests roughly equal or more manpower in indirect fire, they will beat it. But the mortar pits strength is that it can take hits while still dishing them out, and can even be repaired while doing so.
The best way to beat a mortar pit is to get within the minimum range with anything that does raw damage. MG42's are remarkably good up close. If you're just lobbing mortars/ISG's at him you're wasting time and not using those units very effectively. (Plus if he can shoot back you're not going to win that fight.)