I've floated anubias in a QT long term, but they will eventually attach to anything they can: filter intake, prefilter, other plants, wood, rocks, each other, etc. If they are in current, they may grow into a ball shape, not terrible, but a bit challenging when it's time to permanently "plant" them. But you can trim roots a bit &/or rhizome.
This is mostly for A. nana, nana petite; the larger leaved ones (barterii etc) really don't work as well free floating. If the roots of these (or nana) are long enough, you can park a smallish rock on the roots to hold them in place on the substrate & keep the "ball effect" from occurring...much harder to deal with on larger plants unless your tank is pretty big. & like Snake Ice said, do not bury the rhizome!