Compelling

It's an adjective with an omitted implied noun. The combination "one of the most + noun" is not possible.

onizoIs it an adjective here too with the omission of a noun or used as a noun?

The definite article can transform an adjective into a nominal. The subject noun must be inferred from context.

The poorest are those who live on $1 per day.
"Survival of the fittest" was coined after Darwin published his famous book, Origin of Specie

AlpheccaStarsThe definite article can transform an adjective into a nominal

Do you mean you think this can apply in the case of "one of the most compelling is that it ..."?

Yes. "the most compelling" is the complement of the preposition "of."

Hmm, I don't know if I am imagining it, but I see a difference between the function of "the most compelling" in "One of the most compelling is that it is ..." and the function of, say, "the poorest" in "The poorest are those who live on $1 per day". For me, the former has a greater need of the implied noun than the latter.

'One of the most compelling" works for me only if we have a noun in the preceding sentence, for example "There are many reasons for Sanders's sucess".

fivejedjon'One of the most compelling" works for me only if we have a noun in the preceding sentence, for example "There are many reasons for Sanders's sucess".

Yes, I would agree. I think this reflects the difference that I perceive.