With regard to flying the FTDK in IFR.
One of the problems is that they are damned near impossible to trim. With the V-tail, you can set the elevator trim so the ruddervators are in trail, which reduces drag, but as soon as you try to trim for yaw, they go out of trail! It's a hit-and-miss process and takes a lot of experimentation to get it right. Personally, I don't think I've ever done it.
With regard to rudder input only.
It will behave like any other aircraft, you will get a roll. A few years ago a friend got in strife when he lost control over the right aileron due to a cable end fracture. It resulted in the starboard aileron deflecting fully down and locking there. He pulled off an emergency landing safely with skillful use of the ruddervators and throttle. Had he not been able to counter the savage left roll tendency with rudder, he wouldn't have gotten it down.
Walrus