If you get a decent set of flat pedals with pins, your feet aren't going to slip off...at least not easily.
Indeed. I've got a set of Crank Bros Stamp 7s on my MTB that I'm going to try out on the gravel bike. I've got Saint MX80s on all of my other bikes. The Stamps are in a completely different league.
I tend not to cycle in mud or rain so losing grip is slightly less of an issue.
However, in the back of my mind is my previous failed attempt at going clipless and it's something I want to conquer eventually.