Honestly, this would be so unfair and rage-inducing. If a engi got a sentry that a spy couldnt sap, it really efects, and slows down the enemy team. While it would be interesting to see gameplay with this, I honestly think that it would be a little unfair. And im just talking about one of these. Imagine a rooftop nest of these that would be almost impossible to take down. They could be placed in difficult positions to reach, and make gameplay so crazy and unfair. If this was added to the game, It's stats should be lowerd, by like a ton maybe?
Maybe something like:

Can be placed on flat walls and roofs.
-25% ammunation
-50% damage output
Can not be repaired

That's my idea for stats, if this was added into the game. I don't hate this weapon, and I honestly think it's a geat weapon. But if it had the same stats as a normal sentry, it would be devestating gameplay. Thanks for your time reading this.

No h8 plz,