Board Thread:General Discussion v2.0/@comment-26347196-20151109173852/@comment-26347196-20151115070741

Funny enough, in the Pacifist route, Flowey was able to consume Sans's soul (Without his retaliation, at the very least). So I think, plot-wise, Flowey may be stronger than Sans?

But if I want to deny this fact, I can think of a theory that Sans's power is too evil and merciless, he had to hide it away for sake of Frisk's purity (considering Frisk is taking a Pacifist route)