So the massive drawback of fish is it's incompatible with bash (and other shells people use). If you're in fish, this is easily remedied because almost every machine has bash too. However, you can't expect another machine to have fish.
However, I just really like fish. I also live out of my own shell, and I don't spend much of my life remote-ing into other machines. When I do, my mind flips into bash just as easily as I flit from TypeScript to C#. To me, this is largely a non-issue, aside from when I need to work out how to set the `$SHELL` variable for a program that expects bash.
There is no need to change `$SHELL` :-/ you can use Bass (https://github.com/edc/bass) to execute any bash script and it will apply the environment variables to the current fish environment.