What are the advantages and disadvantages of ruddervators?
More induced drag 

A disadvantage (especially critical during landing) is that there is more induced drag to make certain movements.
The image below helps to understand this:



Source

To yaw, you have to use both ruddervators (contrary to just a single rudder movement for a normal tail). This is done in order to compensate the unwanted vertical component. This leads to increased induced drag, which is a big problem during take-off.

Less interference drag

The number of intersections goes down. In the case of a conventional tail you have three intersections (fuselage - left elevator, fuselage - rudder, fuselage - right elevator), whereas for a V-tail you only have two (fuselage - left ruddervator, fuselage - right ruddervator). This reduces the interference drag.