Purely my speculation but...seems all these guys have a shelf life and will run into players that they just cant figure out how to correct, fix, or maintain. Like, sure, he was the hitting coach for some great offenses, but... the players that were in those offenses are largely gone, with the exception of Vlad, Bo and Kirk, all of whom underwent fairly drastic reductions in their offensive profiles over the past 2 seasons, with now some resurgence on 2 of the 3, and the 3rd can be largely attributed to injury issues.
So, it's hard to fully put it on Guillermo, but at the same time... why did it take 2 seasons to "fix" Vlad? Why did it take a ton of time to "fix" Kirk (if indeed he actually is..) How much credit does he get for Horwitz, or any of the buffalo guys when it seems that Matt Hague was largely responsible for them?
Its hard to say, I think it just comes down to him having lost the ability to communicate what he wants to communicate to the players he needs to communicate it to. it's probably not that he's giving guys the wrong advice, he's just giving it in a way that they aren't getting. Maybe it took him too long to identify the mechanical problems with the aforementioned players... maybe he knew exactly what the problem was but couldn't explain the fix right. Who knows?
At the end of the day, the offense has largely been declining for 2 straight seasons, so.. at some point he's going to get the blame. Some of it is on him, some of it is on the FO for trading away good hitters and not bringing back good hitters. Nature of the beast.