I really don't like this, since it makes shield a blind shot, for the chance of making an attack miss. Wich is not a big deal for characters with high AC (since +5 is enough to make almost all attacks, that would otherwise hit, miss instead), but for characters with low AC it is. So the nerf doesn't really work well.

Besides: It makes it even harder to do something I really like: figure out things about the monster by ,,reading" their roles and thus adapting my characters strategy.

And lastly it makes the PCs feel... babysitted, since the DM does not seem to trust them and just plays the whole thing for them. (Why even bother rolling any check yourself if the DM can just do it all the time?)