In terms of the underlying bit assingments, bitfields have been pretty safe for a very long time now, as you can assume little-endian. Just a question of bearing in mind the packing rules, not too difficult given stdint.h. I went full bitfield shortly after the time I went full size_t, about 15 years ago now.
Big-endian systems sort-of still exist (I do indeed have an Amiga, and an Atari STe, and an Xbox 360, and they all work, so they must count) - but they're only slightly less weird from a practical C programming perspective than stuff like the DSPs with CHAR_BIT==24, and Harvard architecture microcontrollers with 2-byte ints. And so you can ignore them. People that use still actually these systems will be used to it.
Big-endian systems sort-of still exist (I do indeed have an Amiga, and an Atari STe, and an Xbox 360, and they all work, so they must count) - but they're only slightly less weird from a practical C programming perspective than stuff like the DSPs with CHAR_BIT==24, and Harvard architecture microcontrollers with 2-byte ints. And so you can ignore them. People that use still actually these systems will be used to it.