abirdcall
(she/her)
It actually IS a weakness of Silvery Barbs. The spell is as good or as bad as the spell it enhances. It's reliant on a combo effect - that's not a strength.
That's like saying any spell or ability is bad if it relies on a saving throw.
You said it yourself - the spell is as good or as bad as the spell it enhances is exactly right and that is why it is broken. Banishment is a 4th level spell and here is a 1st level spell that is as good as Banishment. Only better because it is just a reaction instead of an action and also grants advantage to an ally on a saving throw. So we have a 1st level spell that is significantly better than a 4th level one (which is considered to be a great spell).
It's broken.