UHMMM you're contradicting yourself dude. "Most compatible Nix clone" with what, itself? That's not compatibility. That's Microsoft-style compatibility. And yea if you write something on Linux it probably shits itself on Solaris, that's because Linux and GNU in general hold your hand and make your life easier. People take simple stuff like grep -q for granted... I choose Linux specifically for this reason, and yes I love it too dude but try writing your shit on Solaris or FreeBSD, THEN moving it to Linux. Chances are it will work flawlessly. I don't just mean some sh-glue, I'm talking even freaking C code. I've personally witnessed Linux (well really gnu libs) let off-by-one errors happen in run time without segfaulting, Solaris totally says 'fuck you' immediately, and you get to fix your code so that it is actually right. Luckily, I don't worry about mixed environments anymore and I am that asshole that uses tons of bashisms and gnu-specific stuff everywhere.
No amount of careful planning will ever replace dumb luck.