Ultralights with slight dihedral will roll and yaw on rudder input (e.g the Bloop 3, Maxair Hummer) so some don't have ailerons for the sake of simplicity. Is this kind of control input advisable, or are there disadvantages?
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."
Rudder only limits banking as described very well by John K, and might be a distorted view of increased safety by helping the pilot to not become disorientated. It may also be an attempt to not over stress the structure or prevent a stall in a high G turn.
After considering John K's comment it appears to me this is more a marketing or cost savings stunt, then a safety consideration.
A stall in a skid or slip is all that is necessary to enter a spin! So, essentially you are trading a less hazardous situation (disorientation) for the biggest pilot killer on the planet! (spin)
I have some tens of hours flight experience with radio control aircraft that fly this way -- throttle, elevator, and rudder. How well it works depends very much on how much rudder authority you have, and how much dihedral.
I have seen and flown aerobatics with these no-aileron aircraft, including prolonged, controlled inverted flight (tricky, because yaw-roll coupling reverses when inverted), outside snap rolls (again, roll is reversed relative to rudder and the aircraft may tumble if the wing unstalls before rudder is neutralized), as well as more conventional slow rolls, chandelles, Immelman turns, etc.
I wouldn't expect an ultralight to be flown through any of these maneuvers (at the least, you'd need full aerobatic stress limits), but I would expect one to fly well in normal operation if the rudder authority and dihedral are correctly balanced. If you're interested in building one and don't want to follow an existing design, I'd very strongly recommend building a radio control test model to verify the setup (tail moment, surface deflection, CG position, thrust axis, etc.) before cutting any tubing or fabric.