The Short Circuit is OP because it shuts down two counters of the Engie - Soldier and Demoman.
Demos get it worse because 2 out of 3 of their weapons are projectile based and get destroyed by the Short Circuit - Soldiers with Banners or Boots get it too. Now, you need to rely on melee or shotgun to get things done.
To make things harder, most Engies will have a sentry or more commonly a minisentry, that thing has a high range and a messed up firing speed, and it boils down to...
1 - you win against the engie, but the sentry kills you
2 - you destroy the sentry with your shotgun, but the engie on the other side deals damage to you
3 - you escape with low health and guess what the Engie can outrace you and chase you down till you die
4 - by a stroke of luck you destroy both
This is why it is OP.