It is a first level spell that can force a spell of any level through. Even if only once a day you use it for force through a banishment or polymorph, it is as if you had another 4th level spell + an extra action (not having to cast again) + a free advantage. It is insanely OP.
This is made far worse by the fact that 5e has very poorly scaling resistances. High level casters are very hard to resist. If you can add disadvantage when you need to, they are way too likely to push save-or-suck spells through.
Is this based on seeing it in play or in the white room?
I have seen it played in multiple groups, both as a DM and as a player and I played a wizard who had it myself and I don't think it is the most powerful 1st level spell by a longshot. Shield is WAY more powerful.
I would put SB as a solid spell, roughly on par with Cause Fear, Absorb Elements, Cure Wounds, Sanctuary and Dissonant Whispers.
It is substantially behind the best 1st level spells including Shield, Bless, Goodberry, Protection from Evil and Good, Catapult, Healing Word, Hex and Faerie Fire.
The biggest problem with Silvery Barbs is most of the time you try to use it to "force a spell" it does not actually change the outcome. All it does is cost a 1st level slot, give someone an advantage and worst of all use your raction. That is not all the time, but it is most of the time. The major weakness in Silvery Barbs is it is a reroll on a save or attack that has already landed. This means statistically you get to use it most is when the enemy has a high attack bonus or a high saving throw and that enemy is likely to make the reroll, while taking your reaction which can be crippling, especially if you are a wizard with truly game-changing reaction spells like counterspell, absorb elements and shield, which always land (or in the case of shield almost always).
The last Wizard I played with it went to 14th level. We had three casters in the party with SB (a bard, an Artificer-Wizard and me). The party was effective with Silvery Barbs changing a save on a spell exactly twice in that entire campaign and once it required all three of us to cast it to get to that as he saved and saved again and saved again and then failed (we used it to land Psychic Lance on Jubilex who was already out of legendaries).
We did use it to undo critical hits a lot and it is great in that role. We also used it to successfully cause someone to fail a hide check once and a grapple check once. We also used it on our own Barbarian to make him reroll an attack and give our Rogue advantage on a save against charm that he got to make at the end of his upcoming turn. That said, the only time my Wizard went down that entire campaign was when she used SB to undue a crit - the bad guy still hit, then hit 2 more times, then his ally hit me twice and I went to 0. If I had used shield instead I might have survived even after taking the crit.
It is a solid spell, but after extensive play it has not been close to OP at any table I have seen it on.
