What has one fusion got over the other ?
Chakra - no
Genjutsu weakness - no
Taijutsu prowess - no
Ninjutsu prowess - Debatable
So what exactly are your reasons ?
I'll keep this short, but I'll answer any of your questions if you need more detail. Skimming here.
Chakra -
Hashirama destroys Kurama's chakra, so Naruto is out there. With senjutsu enhanced Kurama one could debate he could go farther, but with Madara's PS added with Hashirama's wood. It becomes a rape.
Anything EMS Sasuke can do, Madara can do it better. The only thing Sasuke has on the upperhand is Amaterasu, but Hashirama can sense that coming and easily block it.
Genjutsu - Out of the question because of Sharingan users, and Naruto would be a Perfect Jin so any attempt there is a joke.
Taijutsu Prowess -
Naruto > Sasuke in taijutsu
Madara > Hashirama in taijutsu (by feats)
Madara casually owns Naruto in taijutsu, so even with Sasuke added, Madara takes this .
Ninjutsu Prowess -
We could go on for days about that. Let's just take it into account that Hashirama/Madara are stronger than BSM Naruto/EMS Sasuke
separated. So with both of them combined, it would not make sense for HashiMad to be weaker than NaruSasu