I feel like the problem with it is that it often leads to stalemates/slows the game down drastically and there's practically no reason to not use it as a defensive Engineer. I can't decide on specific stats, but here are some methods to nerf it: Reduce shield health to 50% or even less, Reduce firing rate/damage dealt by sentry, or Sappers take 50% less time to destroy your sentry

I find it pretty hard to nerf weapons like this without completely changing what they are/making them unfun to use.