What are the disadvantages of using just rudder to roll an airplane?
It works in a half-assed way but the key word is half-assed. You'll always be skidding around the sky since sideslip is required to obtain and maintain any rolling moment.  Control response can be somewhat laggy, since you have to induce a skid and wait for the roll, depending on how much dihedral you have.  So to have something approaching snappy control response, you need way more dihedral than is necessary for normal lateral stability.  The inability to cross control limits the ability to handle crosswinds and prevents you from using sideslip to increase descent rate.

So it works after a fashion, but will never be as good as full 3 axis.  The thing with ultralights is that because of the really low mass/large surface area, they are really strongly affected by turbulence, and when close to the ground, half-assed control authority is the LAST thing you want.

Control authority is not a place I would cut corners in the interest of simplicity.  I think that the attraction of 2 axis control for ultralight designers was in the idea that non-pilots getting into ULs would think "Hey that must be only 2/3rds as difficult to learn as 3 axis.  Must be better."