The problem with 12 speed mechanical shifting is that is needs to be tuned exactly right.
Electronic you can set and it will stay indexed pretty much forever (unless you give it a whack obviously). This makes 12 speed a good idea - more gears and no extra hassle.
However, on mechanical shifting, the tolerances are so tiny, that it really must be dialled in properly. This also means that the slightest change in cable tension/alignment/indexing can throw the entire thing off.
And as for the levers, having ridden both Shimano and Campagnolo, Campag thumb button and paddle system simply cannot be beaten.