Level 1+
Becuase it runs on electricity, it will slowly run out of universal ammo.. Alternatively, another building (dispenser, teleporter) must be near it so it can use some of it's electricity to run. A medic can heal the sentry, but instead of repairing it he will supply electricity. Ubercharges will increase the re-energization rate. (it is extremely low)

Level 2+
Due to the fewer barrels on the sentry, it will suffer lower firing speed.

Level 3
The sentry will use a burst of ammo to shoot a fast firing lazer that doenst deal damage and doesnt explode, but can pierce enemies and light them on fire. Costs half of the electricity to fire this shot, but will only shoot if the electricity level is 75% full

Too OP?
Too MANY stats?