FinalCut Pro X can’t export OMFs, you have to use 3rd party software. I’ve never had luck importing from those OMFs. FCP6 and 7 have worked fine for me in all versions of Nuendo so far.
One problem I run in to with exports from Avid systems is audio associated with Canon 5D video. Maybe this is related?
OMFs are as old as dirt now, so I’m not sure how they would handle the newer video formats and timecodes. Maybe the project is at an exotic time code?