FormerlyHemlock
Hero
Now it is indeed true that both checks are Int checks, so a creature with a low Int is very susceptable, however, that said, imagine a creature with a 10 Int, with no investigation proficency, against a 3rd level caster with a 16 Int. That is a DC of 13 if I'm not mistaken. 2 chances at +0 to defeat DC 13 if we are to crunch the math is 64% in other words, 36% for the spell to stick. If opponent has an 8 Int (-1), that becomes a 58%. If Int is 6 then 51%. In other words, against a very, very low Int creature, it's basically a coin toss for the spell to work at all. That's my concern with the spell. It's not that it's an Int save, it's like an Int save with advantage.
It's better than that, because it's easier to penalize ability checks than saving throws, e.g. via Cutting Words--and besides, investigating still burns an action in one way or another.
For a 13th level PC going up against an Adult Red Dragon, if the PC has a DC of 18, then the Red Dragon's +3 to Int rolls gives it a 70% chance of failing the initial save, and then a 70% chance of failing the Int (Investigation) check, so a 49% chance of being fully affected (barring use of Legendary Resistance). That compares favorably with its 50% chance of making a Wisdom save (next weakest save) against e.g. Slow, and a 50% chance of the Slow wearing off in the next round. (49% vs. 25% chance to affect it for more than one round.) That 49% approaches 70% if there's a Lore Bard around with Cutting Words to penalize the dragon's Investigation check by d10.