Most Predator X's I've seen could go in to wing flutter at very high speed. I like to do a series of high speed vertical dives to get away from them. Even covered in Ultracote, they can flex a lot. I haven't seen an Icrex covered one doing high speed dives yet, so I don't know if it's possible to get one to stop flexing.

fwiw, tape can create as much or more tension as Ultracote if you put it on well. Many tapes will also shrink some with a sealing iron, allowing you to tighten it up some and get it to stick better. Try going over the plane with an iron if the tape is coming unstuck.