It's not overpowered, a unit shouldn't be idle for that long anyways, if you know they have a 120mm mortar, and that they are vet 1, then you shouldn't leave your units idle, and move and re deploy.
not viable. Mortars need to be stationary to fire their barrage, and AT guns don't operate when not still. thats when the 120mm hits, hits onces, and hits HARD.