The U Can Do is known for flutter and other damages in flight. A guy at our club yanked up on his UCD 60 while coming in to land the whole rear section ripped off. The wreckage showed that hot glue was poorly used to build the model. The Funtana 40 had a bad reputation for wings ripping off when flown at moderately high speeds.
Flutter can be induced by uneven air coming around and over control surfaces. Sealing the gap goes a long way in flutter prevention. Also, making sure the surfaces are even along their span helps. A crookedly installed aileron is asking for problems. Then, make sure the linkages are tight and slop free.
I got a Fantasy 40 from a buddy and it fluttered twice. The buddy had lost his first one to elevator flutter and built the one I had. Later, on RCU, the owner of Skyshark wrote about buying the rights to the Fantasy, and sort of became a folk hero for the pictures of Angie holding up the model on his website.
He also mentioned a redesign of the rear formers, etc to prevent chronic elevator flutter problems in the design.