I followed all of the advice I could find about what flexible filaments would work with the Bowden tube of the Flux Delta, and I settled on the pricey Soft PLA from Matter Hackers. I really struggled to get the stuff to load and start a bead melting from the hotend. I ended up using my trusty (greasy) Universal Filament Filter and Lubricator and finally got it in. The first attempts resulted in jams, and the motor and grippers acted like nothing was there, while the filament just ended up all twisted.
After using the most conservative settings listed here, I have it extruding at a snail's pace for the skirt and initial layers. I'm printing the Apple TV Remote bumper that's designed to be printed in Ninjaflex. It seems to be working OK, but it's nerve wracking knowing that the whole thing might gum up if there's any friction. The Bowden tube seems like a significant limitation for many of the exotic PLA variants, and these exciting materials are one of the things that keep me fascinated about FDM. Fortunately, everything seems to work if you can get the right expert settings recipe. AQUA NET is providing sufficient adhesion on the borosilicate glass bed.
Update: The printer stopped extruding about 1/3 of the way through the print. I tried again, but it quit extruding the next time during the first layer. Back to the drawing board.