I had power outage last week when I was testing continous front USB recording, and the wav files were comepletely intact. It was just 2 channel recording, so it doesn't give you gurantee (nor RME will), but something you wanted to know.

The write process is buffered, appending write, so it should be all right, although power surge may corrupt file allocation table, and in such a case, file won't be readable.