I have experienced this phenomenon as well, the latest incident being yesterday with my brand new 10.5" iPad Pro.
My project caused the app to crash and quit unexpectedly the moment it began exporting, regardless of how I wanted to share it.
The problem was the first introductory clip. When I removed it from the project, it exported just fine. This is very irritating because it is an introductory clip that I use on every single one of my projects, from family home video to my professional revenue generating videos.
The clip in question is created in Final Cut Pro on my iMac, and shared via iCloud for my iPad to access it via iMovie. I tried several different setting for export, including "Apple Devices 1080p" and the clip still caused iMovie for iOS to quit unexpectedly on export.
The clip causes zero problems during the editing process. It loads into the project absolutely fine, all transitions can be applied, it can be trimmed, split, titled over, volume adjusted, audio split, everything with no problem. It just won't export.
So...my advice is that of you are able to process and export to ¾ of the way before your crash...look for a clip around that far through your project that may be causing the issue and try removing it. It may be a little bit of trial and error, and extremely irritating, but it is most likely the culprit.
In my case, I exported and then added back the intro clip in Final Cut Pro on my iMac. That kind of defeated the whole purpose of doing the project on the iPad though...