12MHz capable parts was quite normal back on the day. The Motorola range of 68000 chips was 8-12-16MHz. Was only on the end of the 80's decade that the 28MHz 68000 showed up (only in PLCC package, IIRC).
The only disadvantage on the 12MHz version is that you can't push it over the limit: this chip get fried if overclocked. And nothing on Amiga side can push it over 7.09MHz. 16MHz is out of the question.