In addition to being comfortable and genuinely one of the most interesting wireless earbuds I've tested, the UE Fits also sound pretty excellent. I've ran with them a dozen times and worn them on numerous indoor cycling workouts, had no issues with them being uncomfortable or even falling out of my ears.
The idea with the UE Fits's custom mold is that there's no eartip and no pressure point, and therefore more comfortable - and yes, they are really comfortable. It's effective, but it also still relies on a silicone or foam eartip that to hold the earbud in place - which creates a pressure point and could cause discomfort during long listening sessions. Instead of using fancy LED technology like the UE Fits, Decibullz has you create a custom molds using hot water, just like a custom mouthguard. Decibullz, for example, goes about the molding process differently. There are other custom mold wireless earbuds that you can buy besides the UE Fits, and I've tested a few of them. So there's really very little to worry about other than some inconvenience.
But Ultimate Ears promises that if you screw up - which honestly seems pretty hard to do - you can return them free of charge and Ultimate Ears will send you a new pair. Obviously, the biggest fear with this do-it-yourself molding process is that you do it incorrectly and then you're stuck with expensive wireless earbuds that you can't use.