I've been giving #fish a try on a periodic basis every 6 months or so.
The pattern is always the same: I read an amazing article on how fish makes feature X easier/fancier than bash/zsh, I install it again, I spend half a day trying to export my two decades of bash/zsh customizations, and eventually I just give up overwhelmed by the amount of required work.
Fish is a great shell, but I don't know why they decided to go all the way and completely break the compatibility with anything that POSIX has produced over the past four decades.
I won't rewrite all of my shell functions, aliases, if statements, for loops, string concatenations, and/or conditions and environment variables to comply with a shell that is only compliant with itself, sorry. And I don't know why they decided to go the nuclear way and break compatibility so hard where they could have at least guaranteed a back-compatibility layer with (at least) zsh. Reinventing the whole wheel to make it look exactly the way you want, while disregarding compatibility with everything that already exists, is probably the biggest violation of the UNIX philosophy.