Board Thread:Naruto Discussions/@comment-27924662-20170227151559/@comment-732150-20170309034258

It was pretty clear that ninjutsu immunity happens because Yin-Yang Release is used to nullify ninjutsu. Tobirama noticed that Edo Minato's arm wasn't regenerating, and based on Obito's reply about his power, deduced it. That's why they had to switch to senjutsu attacks, because senjutsu doesn't get nullified by YYR. What Six Paths Senjutsu/Six Paths Sage Mode does is grant you the ability to use YYR, you still have to purposefully use YYR to cause the nullification effect. Basically, SPS/SPSM don't outright make you immune to ninjutsu, but rather give you the tool to make that happen.