Rumors say that bondtech is working on a UM3 feeder, this will most probably help for flex materials, also 3dsolex will have a 1.75mm kit soon with the possibility to switch from 2.85 to 1.75mm in a minute or so (and changing the core of course).
I have printed semi flex with the UM3 and NGen flex it prints just fine, but ninjaflex is really hard for bowden setups