I had a run yesterday which (unknown to me, since I joined via QP) had the endless horde modifier active. It was awesome. A new horde spawning right after killing the last one and a constant string of specials. It was fun as hell and I want more of it.
My only complaint there is that modifiers don’t adjust the reward for the run. They really should.
Then later, I selected a map with that modifier and it was just a normal run. No extra enemies. No faster spawns. Nothing. Big letdown.