The newer versions are called bass, and you can compare yourself here: http://byuu.org/tool/bass/
There is full documentation of every possible command, the code is readable by human beings (eg variables are named words instead of single letter abbreviations, C++ is used for more than just the class keyword, there's more than one source code file, I learned how to use the spacebar key in my code, etc), it has a real recursive descent parser that builds a proper abstract syntax tree for evaluations, it has a macro system that supports recursion and conditional evaluation, supports multiple architectures instead of just one, allows the user to dynamically add/change the actual opcode mnemonics used by the assembler (useful as the SNES has games using several strange coprocessors), has no known bugs (as opposed to xkas' that allow native code execution), etc. xkas was written when I had about a years' worth of real programming experience, bass was after over a decade's worth of experience; including experience using assemblers to create code.
If you want to know why RHDN users threw a fit about it; it's because I used // as a comment delimiter instead of ; and decided to add a closing marker to defines so you could say "lda {macro}_b", which was impossible with the old "lda !macro_b" because it'd look for "macro_b" instead of "macro".
That's really it. That's the whole of why many people there say they'll never use it. The syntax is 99% compatible otherwise. I could link you to posts saying as much, but I really don't feel like browsing their site any more than I have to.
SMWC's case is very different: they're all hacking the same game, so they require 100% backward-compatibility with a decade's worth of game patches; and they only make the problem worse as time goes on. This constraint would not apply to new games started on from scratch.
