Our local library makerspace has a Prusa i3 Mk2.5 and has been able to print NinjaFlex with little difficulty. The newer Mk3 model has an improved extruder/filament drive and would be no more likely to have problems, perhaps even less trouble to print flexible filament.

I did a quick read for compatibility of the Prusa i3 Mk3 with these filaments and it is suggested to slow the print speed to about 30 percent (or slower) than one would use for PLA. It is also suggested to use glue stick on the PEI bed for easier removal. The Mk3 does not use a PEI bed, but the glue stick provides a thin layer allowing easier removal, not necessarily better adhesion.

Consider that the Prusa in kit form also provides you with a solid understanding of the works and makes troubleshooting that much easier. The Mk3 is more easily constructed from a kit than the Mk2.5 and provides additional features not to be overlooked. I assisted in the assembly of the library printer and would have loved to have had the Mk3 in place of the Mk2.5 for all those features as well as easier assembly.

I think you'll find you can't go wrong with a Prusa.