It seems that Nuendo’s (and Cubase) problem with the FLAC format is not limited to recording: when making an Audio Mixdown Export to FLAC, the problem is quite similar to using FLAC as a record format.
I exported a 90 minutes mixdown to FLAC that took 30 minutes to export. When the export was finished, the export function hanged in there for another 15 minutes before freeing up Nuendo.
Clearly, there’s something going on with the FLAC implementation that’s really suboptimal…