My main complaints about UNET could be summarized as "the order in which things happen is very chaotic" (in unbelievably many ways!). I've not checked to what degree Mirror has improved on this.

Based on a superficial look, I'm worried it might have been more concerned with adding features than fixing and simplifying the fundamentals, but I don't really know.

Mirror is amazing :) But it's a big shame for Unity that you need to use it in the first place, because they don't ship a working networking stack out of the box.