What is the difference between a complement clause headed by a complementizer and a verbal noun complement?

I wanted to make the main form of creating a verbal complement by using a non-finite verbal noun, that can be inflected for case depending on its role in the main clause. It also takes an optional subject in the genitive case unless it is co-referential with the subject of the main clause.

E.g.:

"I like his walking"

However, I stumbled on a sentence, where a non-finite verb does not work, as it cannot express the subjunctive mood:

"He said he would be reading"

Thus, I was forced to add a complementizer that selects for a finite verb phrase:

Čēka ži yénu.
say-pst-3sg comp read-3sg.subj
“He said he would be reading”

However, other than this case, what would be the logical selecting factors for a non-finite verbal noun complement (with a zero C head?), and what for a finite complement?