It's not difficult to customize. IMO, it's actually easier to customize yourself than bash/zsh because I find the fish syntax much more natural than bash, but YMMV.
That said, I believe that zsh has a bigger community, and therefore most likely a richer ecosystem with more plugins. For example, oh-my-zsh has 142k stars, and oh-my-fish has 8k stars.
However, even with the rich ecosystem of plugins, I wasn't able to get my zsh to be as pleasant to use as my fish, where I only have some self-made customizations (not even using oh-my-fish).
But if having lots of plugins to choose from is important to you, zsh may be a better choice.
I'm a previous bash user who now uses fish and loves to customize everything, and compared to bash, customizing fish is a joy. Its so much easier to do, using oh-my-fish brings a great selection of plugins (admittedly lots of them are inspired or clones of oh-my-zsh plugins) and writing plugins is a breeze as well.
That said, I believe that zsh has a bigger community, and therefore most likely a richer ecosystem with more plugins. For example, oh-my-zsh has 142k stars, and oh-my-fish has 8k stars.
However, even with the rich ecosystem of plugins, I wasn't able to get my zsh to be as pleasant to use as my fish, where I only have some self-made customizations (not even using oh-my-fish).
But if having lots of plugins to choose from is important to you, zsh may be a better choice.