The biggest issue I've had when attempting to customise my shell is the same issue I've had while working with my custom vim setup and that is the issue of a different machine.

When I log into a new machine I still need to be able to find my way around, I still need to be able to edit files, I still need to be able to run various different commands. These can even be completely different shells or different versions of vi (nvi on FBSD for example, vim without python support on another system and so on).

Yes, my workstation has my vim customisations, but it only has the bare needed ones the rest are stock. Even on a vim without any of the scripts I have I would be proficient and I wouldn't miss a beat. Same thing for the shell, typing commands in full means I don't have to worry about muscle memory.

ll for example for ls -la is nice and all, but it isn't available on most of the machines I administer so I end up typing out ls -la anyway. That has become muscle memory for me now, and I am not worried about trying to change that.