6 stars just make it faster.
But good 5 stars make it entirely possible.
I'd argue that they're impossible without Kabuto, though.
Well yeah, you definitely need Kabuto if you're taking 5*'s.
6*'s allow you to brute force to a certain extent on some of the missions without an healer, which is what I've been doing.
Sadly I only have Sakura and she's the worst healer by far since the healing ult is probably the best part, which she doesn't have. So that severely limits what I can do.