the same can be said for almost any other "X is the weakest, but I'm the strongest using it" story; very rarely is it executed in a reasonable or believable manner
iirc in this case, it's because every other debuff user in this world has low success rate in applying the status effects, but even then, it feels like it's just a poor excuse unless they're so low that it's just faster and safer to kill things conventionally.
This is one that I think works entirely because the Goddess didn't even let the MC try out his skills.
I've seen others where the MC uses his skills a few times and they're pathetic, but then they're suddenly goated the second he's alone after being exiled. While I've never read Failure Frame's Light Novel, it really feels like the Goddess revels in the slaughter and torture. Heck, she's outright petty (and I'd dare say petulant) about being asked to use her healing on some of the Heroes that are "needed" to fight the Demon Lord. She doesn't expect these teens to win, she expects them to either die of cockiness, suffer mental breakdowns and die in fear, or slaughter each other with impulsive infighting. All for her amusement. Status debuffs are too good at leveling the battlefield's balance for her to allow.
After all, she seems to have exiled people with any kind of skill that isn't some form direct damage. Sages, Witches, Debuffers—I don't remember any Healers among the summoned Heroes either, but it wouldn't surprise me if any Healers that were summoned in the past conveniently died in their first battle to serve as a martyr for the rest of that crop of "heroes" to swear vengeance for, before marching to their own slaughter.