I really enjoyed using fish for a short while, but soon I learned that it was not POSIX compliant. This was a deal breaker for me since my colleagues would often share scripts that wouldn't just work for me. I use zsh nowadays.
I'm unsure whether minor differences that catch you out occasionally are worse than huge differences that you always have to think about. For me zsh still wins ;)
I think your comment will save my sorry ass one day.
It's not that it's impossible to be a shell polyglot, but GP deemed it easier to learn !fish.