Its a Section A question, so looking for a "one or two word or short phrase" answer.
The first problem I have is - disadvantage compared to what?
Swarming?
The old Q continuing?
Or requeening by the beekeeper? Aaah …
Supercedure often happens late in the season, but not always, and so late season mating isn't a general disadvantage of supercedure.
The possibility of a brood break (even a permanent one) exists if old Q is dispatched before new Q is laying. But they can coexist for months sometimes. So not the generality of supercedure.
I think they probably are considering the case when there are a few Q cells, and the beek (and the bees themselves) must decide whether they are superceding or swarming. Sometimes they can swarm from a very few Q cells rather than a dozen or more. Allowing potential supercedure QCs to proceed does carry some risk that they might just decide to swarm.
Beekeeper stepping in to abort supercedure and requeen with known-good Q takes away that (and poor mating and brood-break) risk.
So I'm offering as my answer -
"Some risk they just might swarm instead of superceding"
Bit of a stinker for a 1-minute question … (its taken me 24 hours to work out what the question implied
)