> So, if anything, it is GNU that is "different" and "incompatible". (Not necessarily a bad thing, mind you)

What I mean under "incompatible" is not-supporting everything that the other does. Just couple of trivial examples: `sed -i 's/require/include/' Rakefile` throws `invalid command code R` Mac (you must use `sed -i "" 's/require/include/' Rakefile` instead) and `dd` does not support `status=progress` on Mac so you have to pipe it through `pv` and this is not free in terms of performance.