The countback has happened a few times in franchise cricket, dating back to 2010. Jimmy Neesham's been on the winning side in one of them.Seems obvious that if runs are tied, wickets become the deciding factor.
A lot more authentic and meaningful that most boundaries - which is a pretty crap approach.
The point of the super over is to decide the winner, and theoretically tied super overs could go on forever so you have to have some point at which they stop and something else decides it.
Personally, I'd say that a tied super over should be decided based on something that happens in the super over, not throughout the full match. If it's wickets, boundaries, whatever, not particularly fussed, but I prefer that the super over effectively starts at nil-all.