Biggest drawbacks for me is mixing up a few bits of syntax and some software doesn't ship Fish completion scripts, although I never write scripts in Fish which reduces the syntax I actually use.
In my opinion Fish is worth it just for the auto complete suggestions alone, but I also really like the way it handles editing config, it's sane default config, understandable error messages, and plug-ins.
Nushell is very nice, but I find it doesn't match the usability of Fish for interactive shells. Love it for scripts though.
I switched to fish because for most interactive use case it’s so much better, without requiring any new muscle memory. I tried zsh at first but it winds up slow to get even close to where fish is out of the box. I still end up scripting in bash or sh for portability.
nu-shell won’t be available in as many places, and it looks to me like it would change paradigm in a way that would hinder working with bash or zsh over time. That’s just my 2c though as I have only briefly glanced at it and haven’t truly considered using it.
I also prefer fish's scripting syntax (and built-in utilities) over bash, and some people don't like installing fish to run my scripts.
Nushell is far from fish in terms of interactive usage, and I find the scripting language to be too unstable for writing scripts you want to keep in the medium term.