So is it all technique or does it also depend on the quality of the cue or both etc.?
It's a little of both for overhand. But it really depends on the person. Body clearance/flexibility/etc is different for everyone. I'm not going to say the quality of the cue matters a whole lot, most jump cues will jump. The newer 3 piece ones are used by a large part of the pros nowadays. Notice most of them use overhand, so there has to be something to having the extra length of the 3 piece that is useful. But... If you watch on some shots, like the ones where dart style make sense, they will remove the extra end to transform it into a more traditional jump cue and dart with it.
But jumping alone is mostly about technique. You will also find that it's easier to jump on heavier tables with 1" slate. For example I find it tougher to jump on a Valley than on an Olhausen, Brunswick or a Diamond. The Valley has thinner slate and often you can feel the whole table shake when you jump (vibrations absorbing the force instead of transferring it). It doesn't mean they are bad tables, but like anything pool you have to adapt a bit.