Nah...he never had great reviews. You can check by yourself, he doesn't have a single album with critical acclaim (in comparison, all of kendrick albums have earned universal acclaim status, meaning they all have higher score than 81/100 on metacritic, with 2 of them being the 2 highest rated rap albums by critics ever). The fact that Eminem albums got panned by critics was actually a discussion among Em fans, they were saying the critics were against him.
As for his grammy awards, thats cool, I agree that 15 grammy is a lot...but...
You decided Metacritic the be all end all because it suit your argument, but it don't capture anywhere close to all the published reviews. And MMLP for example got plenty strong reviews and year end list placements, we not gonna pretend that album got weak reviews.
Critical acclaim ain't objective anyway. It's literally the opinion of (mostly white) critics and most of them come from a perspective outside hip hop culture. Industry awards ain't objective either. It's political, biased as fukk and opinion based with, again, a heavily white and non hip hop slant.
Eminem has won the Grammy for Best Rap Album more times than anyone else. Facts. You dismissive of that now but it will be a big deal if Kendrick ties or passes him, right? And Em easily the highest selling rapper of all time, but sales only matter when they in your favor. So none of this is really objective. There ain't an objective measure of the quality of music.
This is a hip hop board. If we can't formulate and articulate our own opinions what we even doing here?