Specifically, all but one creature in the undead category are immune to poison - so if your dm like zombies (or just dislikes using intelligent enemies all the time because you always end up talking and the fighters getting frustrated because they can't fight) , this cantrip will be effectively useless for you. If undead were merely resistant this wouldn't be a big deal because cantrips aren't about big damage anyways.While there is nothing wrong with it per se, there are other cantrips that mechanically are better IMO.
As others have said, Poison is a common resistance (along with Fire) and quite a few do have immunity. It compensates for that with the d12 a bit, but also relies on a CON save, which CON is one of the highest ability scores overall in the game. Since creatures with high HP will likely have good CON scores, that d12 damage counts for even less.
But, all that being said, if you want to take it, go for it! It will still prove useful enough and you don't always have to take the "optimal" choice to have fun.
Then it would remind me of something like a minor Tidal Wave.I feel like it would be better (if perhaps a bit OP) if it was a 10' line (which would hit a 5x10 rectangle) instead. CON or nothing is pretty rough.... although I suppose it compares favorably to Sacred Flame??
That is significantly under the power of a 1st level spell. It would be a stronger cantrip, though. If we dropped the damage to d8 and made it a 10' line, I think it would be competitive, but not a near automatic take like EB and TtD.That feels more like a level 1 spell than a cantrip.