From my experience this upgrade is safe to do.
I always do the upgrades from a command line and never had problems.
The only problem that might occur (I never experienced though, but I saw on the forum ) is the front-page problem.
I have to outline that none of the servers that I upgraded had fp problems , but if others experienced this then it has to be something in this.
Frontpage works for you ? they have stopped offering the extensions on 8.1 personally I think they should have not since not everone is going to drop using frontpage all of a sudden for microsoft new publisher people are still going to hang on the fp for a while..