This is a specific problem that I am not really aware of. But the fact that this issue concerns e-mountain bike motors in particular makes me think immediately of the use of high-pressure cleaners. These are not yet good for bicycles, but still less for e-bikes. A water pressure of 100 bar washes out any grease and undermines every sealing. I have therefore always advised against using high-pressure cleaners on bikes.
Are current e-mountain bike drives sufficiently protected against moisture?
In terms of operational safety which also includes protection against moisture, drive manufacturers are usually rather modest to be on the safe side in case of damage. The conservative indication of IPX-4 that offers protection from a splash of water may well mean that the drive will survive, if shortly submerged when passing a stream. You can, however, never rely upon that as user. Your purchase decision should therefore not depend on the indications with regard to the waterproofness of a system.
Is such a generally formulated protection rating like the IP standard good at all to comprise the complex special topic e-mountain bike?
This is the general problem with standards. How are you to define a water jet from the garden hose, if you don’t have a constant fluid under laboratory conditions. For this reason it is so important to have well-researched and practice-oriented instructions for cleaning and care in the special-interest press.