The "part-fiend" thing is really hard to pin down, because a LOT of humans have fiend blood. Anyone who is susceptible to the curse is descended from the fiends. The Hayabusa ninjas were also blessed by the ancient dragon deities, and it may be possible that this has given them superhuman advantageous. But, none of this is proven. In other words, we don't know what abilities Ryu/Rachel exhibit from some sort of genetic advantage and what abilities they simply learned through rigorous training/exposure to proper instruction (Momiji being no different, for the record). The same argument could also be made for the Mugen Tenshin ninjas. We really don't know what abilities are inherited through their clan and which ones they are simply disciplined enough to use, if there is any disctinction at all. Thus, calling them the "human ninjas" may be premature, as they may be no more human than Ryu. The possibility that the Mugen Tenshin carry fiend blood is quite high, considering how many people appear susceptible to the curse. Furthermore, the Mugen Tenshin may have additional "superhuman" advantages, much like the Hayabusa's "Dragon Lineage."
All things said and done, it's sorta vague and ambiguous where our hero's powers come from. Only one thing is certain: Hayabusa has the most of them and he kicks everyone's ass.
Now see, you have a point there...except again, it's all just theory.
In NG:B, it's been stated that yes, the Hayabusa Clan has descended from the Dragons that have fought against the Black Dragon, aka, the main monster that was, if I believe, connected to the Fiends and the Vigoor Emperor by proxy. It's also been kind of established that Hayabusa is a Half-Fiend BECAUSE Doku was able to turn him into a crazy fiend within time.
And as we've seen from Alma, it doesn't take too much of a push to turn you into something similar to an inhuman Fiend either, so because Hayabusa didn't turn into something like Alma immediately, just like Rachel, doesn't mean he
can't. You do have a point with Momiji and in fact, I also believe she's of Fiend blood as well, she's more or less in the same territory as Ryu. The problem with the Mugen Tenshin is that nothing of the sort states that they are of the same cloth as the Hayabusa Clan and the only person who was stated to be akin to a demon was Raidou(and Ayane by proxy). So until TN finally decides to go full deep into Mugen Tenshin and Hajimon History...
Kasumi and Hayate are 100% human, Ayane is ???, and Ryu, Rachel and Momiji are 60% Human, 40% Fiend.