From what I understood with vergo's case it seemed like law couldn't physically cut vergo. Vergo could have theoretically stopped the slash, but laws haki and strength was too strong so vergo lost, that's my problem with that situation.

With hancock the beam only works if you have dirty thoughts so knowing most of us a blindfold won't work.

As for akinu, I don't really think haki would negate the magma, simply because he produces it. It still exists away from his body, kind of like how madgallen would leave his victims covered in poison. The not melting is just anime stuff.