On YouTube, "royalty-free" is just another term for "hasn't been uploaded to ContentID yet". If you don't personally own the copyright to the music you use (and sometimes even then), it's only a matter of time before some scammer wanting a quick buck or a troll trying to ruin your life posts a claim and has it taken down, even if the music isn't actually copyrighted at all. Worse, unlike copyright claims, filing a counterclaim is extremely difficult and time-consuming (I'm talking several months at best). There are no penalties whatsoever for filing a false claim, either, and there's literally nothing stopping them from doing it again if you do succeed in getting it reinstated.

Those in charge of YouTube claim they are aware of this issue and are trying to fix it. However, progress seems to be rather slow in this regard.

The best thing you can do, though, is just go for it and hope you don't get unlucky. Enough people manage to do this without harassment that it's still possible for royalty-free or public-domain music to exist. Just be aware that there are a few risks involved.