So I had a similar experience this morning with the IZotope Plug-in demo. It didn't crash FCPX every time but randomly and frequently enough that I uninstalled the plugs-ins and then re-installed them again. That seemed to stabilize things. The Dialogue De-noise module worked very well in both Auto and Manual modes - a definite improvement over SoundSoap 5 or other manual EQ adjustments I've tried - YMMV.
EDIT: Oh, I'm running FCPX 10.2.3, EL Capitan 10.11.3 - late 2013 Mac Pro.
I installed the trial yesterday and played with quite a bit of it. I haven't experienced any problems with it functioning and scrubbing over the thumbnails works fine for me without any crash. The installed audio units worked with my system fine without conflict. Having said that, I agree the dialogue denoiser seems to be useful but the hum reducer doesn't seem to be much of an improvement over the built in FCPX version or other reducers I've tried. I also wasn't overly satisfied with the de-clip plugin. We tried it on one clip that was just barely clipping and it didn't seem to make much difference, but maybe we didn't play around long enough with it. I'll continue testing.