Torb has a fantastic weapon and can make mince meat of a lot of opponents, even doing good damage at range.
The upgrading is there to make turrets not so easy, and punish lazy/hasty placements.
Torb can't upgrade his turret very well because you can outdamage his repair, or stall it so much he can't upgrade (Torb can only upgrade his turret if it has full HP). Then Torb usually just dies for trying to do the thing he is meant to do. Once the offense has made a good presence on the objective, and his turret is down, you can't really put up another turret unless the offense has bad target priority. This is why pro teams only use Torb on first point 2CP and hybrid, because he has the time to actually set-up compared to other defensive objectives and offense.
His damage at range is also inconsistent, his weapon arcs and it is a projectile. He shreds up close with his shotgun, provided he hasn't already been killed because he dies so fast. It also isn't the best for extremely mobile targets because the spread is quite large unless you are point blank range with the enemy.