Pros:
High damage
Long range
Good vs. both infantry and vehicles
In a faction with tons of scouting, vision and artillery support
Cons:
High cost and upkeep
Slow aim and ROF
Low mobility
Slow to vet
I think Pack Howitzer performance is quite good right now and if it receives any buffs a conservative cost reduction from 480 to 450 or an aggressive one like 400 or 420 (err day) will be more than adequate to make it "viable."
Inversely (hehe) the ISG has a reasonable cost and terrible performance.