Well, tbh 5e doesn't even have coup de grace mechanics. Which i personally found one of biggest downsides. Remind me when we was able to sneak up on sleeping cambion. But because of lackluster of coup de grace option, we was just been able barely scratch him with single automatic crit.
And btw by rules you still need roll attack even on sleeping target. So you CAN miss completely.
But that's side notes...
