Type of beadpanel to use for wainscoting in a bathroom
Actual wooden beadboard has been used in bathrooms for decades. It needs to be well primed and painted to do well, and it probably should not be used on a tub surround. MDF shyould be significantly more moisture resistant than wood, again if properly prepared.

There is no question that vinyl would be much more moisture resistant than either wood or MDF. But many people do not like the look of vinyl.

If you application is not especially demanding, you are probably fine with MDF.