As a test, we printed ninjaflex and semiflex on PEI that was not coated with PVA glue stick or blue painters tape as recommended.
Lulzbot Mini, V2 flexy struder, default Cura profiles and temps.
After cooling, it indeed was stuck very well, practically fused in some spots. Definitely use one of the recommended bed treatments!
We found that re-heating the bed to 70C or more allowed much easier removal.
The higher the temp, the easier removal became; however, above 90C or so, both filaments began to soften again, causing the force of removal to warp the part, so lower temps are better. Around 100C+ the filament appears to start leaving a residue on the bed, though we’re not sure the higher temp indeed caused this, or it was just how the print stuck to the bed at that point.