Leigs have the same range as the mortar pit (unless you got one of those light mortar shell bulletins maybe?). Even Ost mortars counter the mortar pit fairly well although IIRC the pit theoretically outranges it. Maybe post patch it will be ok with Leig nerfs but currently in the live version one leig or mortar will shut the pit down and nuke repairing squads.
Obviously other kinds of emplacements are even more vulnerable to indirect.
It takes a fair amount of time for a leig to kill a mortar pit.
And if it's shooting your mortar it isn't pinning your infantry.
Also you can repair between barrages just fine. The only time it's a problem is when they have two leigs.
Also if their range is the same that means mortar can shoot back. Mortar pit will wreck most mortars and non range buffed isg because it can't be decrewed and fires double volleys.
The reason the incendiary barrage is ridiculous is he can barrage off a few shells then drive away and you'll suffer massive damage but assuming the flame dot nerfs apply to mortar halftrack too it should be better.