What issues, if any, should I be concerned with here?

Pro: Does it get you a stable (long-term) tenant?

Con: Who will pay for the marginally extra utilities? How do they filter lint, or does the tenant have to attach a lint trap onto the end of the hose (and change it periodically)? Can the drain hose end up on the floor if left unattended? I have no idea, but that's what I'd be thinking about.

Under the Plumbing Code all residential structures are required to have the following minimum of plumbing fixtures:

1.watercloset
2.Lavatory
3.Bath Tub (some local codes will permit a shower only)
4.Kitchen sink.

A "Residential Multi-family Dwelling" is defined as a residential dwelling with not more than 4 dwelling units and not exceeding 3 stories in height.
In a residential multi-family dwelling the laundry may or may not be shared and it may be at a remote location within the structure, I.E. In a basement or utility room, but if must be accessible to all tenants.

A "Commercial Multi Family Dwelling" is defined as a dwelling unit that has more than 4 individual dwelling units and may exceed 3 stories in height.
Commercial Multi-family dwellings may have the laundry facility in a separate structure but it may not be greater than 500' from the laundry to the furthest dwelling unit.

If you are renting an apartment in your house and it currently has no laundry facility you could be compelled to either install laundry hookups in the apartment or in the basement or a utility room.

The portable washing machines generally have a water hookup similar to those used on portable dishwashers. The aereator is removed from the kithen faucett and a quick connect adapter is screwed on in its place. Both the water supply and drain lines are attached to the adapter so when the unit is discharging water into the sink it is held firmly in place over the sink. Many of them also have a check valve feature that closes the lines unless they are connected to the adapter so they cannot leak on the floor.

The portable dryers are 120v 15A and may be plugged into an outlet in the kitchen. (My wife has one in our kitchen that she uses for small loads of kitchen towels and such).

Permitting the tenant to use those portable devices may cause a slight increase in your water consumption, and you would be justified to charge a small additional fee for water, but even with that, when you consider the cost of setting up a laundry area in the basement or a utility room and providing the tenant a separate access without going through your house, it should prove to be a bargain.