The 14x12 is very popular especially with new beekeepers and the more experienced are turning to it also.
A decade ago they were decidely uncommon. Been around a long time and never became popular. I've been using them for a decade and possibly a bit, I daresay. Can't be sure exactly.
Robin Dartington used them in the middle seventies for his LDH.
Single or double brood. Extra deep or a brood and a half. They all have pros and cons, some more cons than pros. You just need to gather all the information/characteristcs of each format and choose the one that fits your requirements. Try it and see.
For 14 x 12 you only need a removable eke (under a deep box) and a set of frames, to try it out. Reversion to deep, or deeps, or a deep and a shallow simply requires removal of the eke and the frame change. Little actual cost involved, and if you were to change back, even the eke would likely come in handy somewhere sometime.