POSIX compliance is practically a non-issue for fish as it's not intended to be a replacement for bash/shell scripts. It's intended to be an interactive shell (eg. your daily driver) and to that end, it's quite good.
The downside of this is that you do need to "context switch" a little, especially if you've been using bash/zsh for most of your life. I find it easy to differentiate between when I am using the shell as a user interface and when I'm using bash/sh as an interpreter, but for others that might not be as intuitive.
I've also found that since I've stopped using POSIX compliant shells as my daily driver, I tend to "promote" my scripts to a more general purpose language much sooner than I used to when nearly everything could be done in bash.
The downside of this is that you do need to "context switch" a little, especially if you've been using bash/zsh for most of your life. I find it easy to differentiate between when I am using the shell as a user interface and when I'm using bash/sh as an interpreter, but for others that might not be as intuitive.
I've also found that since I've stopped using POSIX compliant shells as my daily driver, I tend to "promote" my scripts to a more general purpose language much sooner than I used to when nearly everything could be done in bash.