I am printing with Polyflex flexible filament. It is a PCTPE filament.
The extruder speed and temperature really dialed in to prevent getting a extruder jam. If you push it too fast, it compresses which shortens it length but increases the diameter which then causes a jam.
The idler being too tight can cause it to bunch up too and not go down into the hot end. Nothing like finding filament coming out the side of the extruder.
If the distance between the hob bolt and the filament guide is too wide, then the flex filament can squish out the sides too. This one of the things that the mods to the extruder tries to address.
Once you get the printer dialed in, printing with flex comes out really well.
This is one of the web articles that I used when I was first starting out with flex:
https://www.matterhackers.com/news/how-to-succeed-when-printing-with-flexible-filament