The two biggest reasons you don't see more people playing 12's have to be tuning and playability. I tune the six strings first, then the others, which makes it a bit easier for me--(I use a tuning fork, not an electronic tuner, which, apparently, is a personal quirk). My guitar, what ever else it lacks, is playable in tune, in closed position all the way up the neck--From listening to old Tex-Mex recordings, it is clear that playing a 12 in tune is a new idea--

I had an old Harmony 12, and that was a much better sounding and more playable instrument than and of the Gibsons or Guilds that used to be so popular, or in fact, anything that passed for a 12 save one of the Martins or the newer Taylors--unfortuately it had an accident, and I haven't found one that anyone wanted to part with since--

Most 12's, even from fairly reputable makers, seem to be just sixes with extra strings, which means that there is a lot of extra tension--