Cas, I don't get what you mean.
"His being in love with you is uncertain."
"It is uncertain that he is in love with you."
Why do you say that a "thing" is uncertain here?
In terms of syntax, the sentence works: Noun clause = Adjective, but in terms of meaning, well, maybe it's just me, but I pick up on something that's quite odd: to be certain or uncertain is to have the ability to weigh possibilities, and things just don't have that faculty.
Who is uncertain that he is love with you?
?That he is in love with you is uncertain.
=> The noun clause is uncertain.
We are uncertain that he is in love with you.
=> We are uncertain.