In my experience I've found inflexibility with respect to OS to be a warning sign that usually means someone isn't going to work well long term as a flexible, open-minded team member. Signs of that at interview time usually push me pretty quickly in a 'no' direction.
Come on. Someone can be an excellent dev but be made much more efficient by their chosen tools/environment.
Come do some public sector work for a while - you'll see what inflexibility really looks like.
Agreed that the worst thing is the muscle memory for shortcuts; my pinkie always needs a few hours before it starts to remember which keyboard layout I'm using.
Aperture used to be one of the two _major_ draws of macOS over Windows for me, but Apple really burned that bridge when they dumped Aperture—arguably the best RAW photography workflow tool on the market—and pushed towards Photos, which lacked about 50% of the features that made Aperture amazing (Loupe, star rating, workflows, UI, shortcuts, etc.).
Pretty much all the software I use is cross-platform, and even Windows is a fairly acceptable Unix nowadays, so the biggest transitioning problem I have is keeping track of which modifier keys to use for my keyboard shortcuts.