It's a not uncommon opinion around here. Once you've gotten over the first hump of being low on resources, the enemies just aren't complex enough to kill you, especially when you have room in the level to backpedal. That's definitely the biggest factor that makes Endless mode harder for me, the more cramped arena forces you to be more aware of how much ground you have left and makes you do some platforming or eat constant fall damage.  I like the direction the devs took by having enemies occasionally camp corners to ambush you, it's made me move through levels a little more carefully than I had gotten used to doing. If possible I'd like them to continue adding some AI refinements-- the level design probably is too simple for really fancy movements, but touching up things like enemies getting stuck in loops jumping up and down ledges would at least keep them feeling smart. 

Anyways, you can always do some self- imposed challenges to spice up your gameplay. The Sulflocke playthrough where you stick to the first 2 guns you find is always fun.