No, it's not B-rank immunity as the jutsu only heals damage, not prevents it. It also doesn't heal 'b-rank damage' only 40 points of damage per turn. So it has no real value unless in using the health system, but in normal battles it would only heal flesh wounds, like it says it won't heal anything too complex. Anything more than cuts, bruises or simple fractures (line breaks in the bone where the bone doesn't separate or protrude the skin) and this mode becomes ineffective.