Naruto avatar survived explosion from both those attack combined. Yet he was untouch by explosion .
Now madara juubi PS would be stronger than kurama avatar .So even if explosion reachs madara that won't be problem.
That is guy whose pure jinchuriki body tanked giant lava rasenshuriken which made explosion bigger than regular tbb it tanked evening elephant and night gain.So his body won't have problem tanking what is left after susanoo tanks most of it.
If susanoo fails then gudoudama or preta path or chou shinra tensei can take care of rest of the explosion.
There is a huge difference between being caught in an explosion and taking it face first. The guy that stands 5 meters
from a grenade is more likely to survive than the guy that gets the grenade tossed into their face but he's wearing
some decent military-grade armor and helmet. Madara might have a stronger defense (not even that much as Sasuke's
BPS isn't going to be a tier below Madara's PS, they should be in the same tier actually) but he's getting it face first.
Not to mention Naruto used wind, which neutralizes lightning, so in one case, the two attacks actually canceled
each other out, while in this case, they are working in conjunction to be a stronger attack.
In other words, the damage that Naruto and Sasuke received <<<<<<<<< the damage that Madara would get.
On the other hand, Madara's susanoo compared to theirs is >= BPS or Ashura Avatar (by going off of the portrayal
that it is BPS's equal). As remember, BPS, is also a rikudo enhanced susanoo fueled by all the bijuus.
Madara gets completely obliterated. Even if he's immortal, the bijuus fueled that attack so the bijuus get ripped out
and Madara's body gets obliterated. In fact, the bijuus might even get destroyed too and would only regenerate
years later.