There's nothing major. I maintain a set of POSIX and fish scripts as part of my job and, given that I have to know both relatively well, I find I end up preferring the on-average-slightly-more-terse POSIX syntax.

One thing that can be particularly annoying is that it seems to be impossible in fish to evaluate a command in a subshell and embed its output into a string without assigning it to a variable first.

ARGV handling is a little weird too. I understand the impulse to get rid of $1, $2, $3, $*, $@, $#, etc., but it ends up just requiring different gymnastics, like writing `set -e argv[1]` instead of `shift.