The suggestion here is that the spell is ONLY useful in these very niche cases....which I will disagree with.
The fact that silvery barbs is most useful if the monster will have a very hard time passing a reroll is obvious.... but that doesn't mean its worthless if the save is a bit harder. The real question is: when is silvery barbs "not worth the slot", aka how high a save bonus should the monster have before its not worth the use of the slot.
True, but it is not likely to be a game changer when used with easy saves.
For example, recently I had a 13th level Ranger fighting from a flying castle against 3 flying Dragons. He fired a bunch of ensnaring strikes at them. They needed I think around a 7 to save and they had advantage because they were Huge. If I had landed one of those they would have plummeted to their death, meaning an instakill. They made every single save and I wasted I think 4 1st and 2 2nd level shots trying to stick that spell over the course of the battle. We won the fight, but those spell slots were irrelevant to it. If I had Barbs through a feat or something, I would have had a bunch of opportunities to use Barbs in that battle. Honestly more opportunities than I had additional slots I think. But more than likely if I had done that I would have completely ran out of spell slots and probably taken a breath weapon to the face at full damage as well.
As it was, I entered the final fight of the day, against Tiamat's Avatar, with few spell slots (albeit 3rd and 4th level mostly). If I had silvery barbs and if I had used silvery barbs, the most likely outcome would have been entering that final fight with no spell slots at all.
Now certainly I could have done a better job conserving slots, but if you are talking about saves, this is the kind of save that is going to come up most often - when the DC is very easy. You are going to generally wait around a long time for an enemy to make a save against a very hard DC, meaning if that is how you plan to use it - turn a save into a fail - you are going to have it prepared and rarely have a chance to use it.
Let's say you are 10th level caster and using disintegrate. You can throw that spell twice a day. Meaning maximum you will have 2 opportunities to use SB with that spell. If you go up against an enemy that needs a 16 to save, chances are you won't even have the opportunity to use SB with that spell at all. Change that to a 5 to save and you will probably be able to use it twice, but it is not likely it will change the result. There is a cost with preparing this spell and if this is the only way you plan to use it, that cost is very high.
Certainly there are other uses of the spell and stopping a crit in particular is awesome and something not easily done by any other spell and by few abilities. But this is where the true power and true utility of the spell is IMO, not on adversary saves. I am not saying it is useless on the later, just that it is not very powerful for this purpose due to the interdependency between the opportunity to use it and the chance to save.