You gain not dying to save your team by hiding away before ultimate. It is called having situational awareness. You track enemy ultimates, and predict when they use it.
It is literally the same thing, except for DPS. You hide until the moment is there, then use the ultimate. The problem you are having is that they don’t ‘instantly kill’ (though some do) the minute you hit Q like mercy did with rez.
The entire premise is the same. Hide and ultimate is something a lot of characters did, and still do.