No, neither one. Here is a clause: "The man runs". Here is what happens when I try to use it as a subject: *"The man runs surprises us." No good.

Here is a noun: "man". Here is what happens when I try to use it as a subject: *"Man runs." No good.

For a subject, you need a noun phrase. You can make a noun phrase from the noun "man" by adding a determiner: "The man runs." You can also make a noun phrase from a clause by adding a complementizer: "That the man runs surprises us."

In some cases, nothing needs to be added to a noun to make a noun phrase which works as a subject: "Men run." Nonetheless, "men" in this sentence is a noun phrase.

No, neither one. Here is a clause: "The man runs". Here is what happens when I try to use it as a subject: *"The man runs surprises us." No good.

Here is a noun: "man". Here is what happens when I try to use it as a subject: *"Man runs." No good.

For a subject, you need a noun phrase. You can make a noun phrase from the noun "man" by adding a determiner: "The man runs." You can also make a noun phrase from a clause by adding a complementizer: "That the man runs surprises us."

In some cases, nothing needs to be added to a noun to make a noun phrase which works as a subject: "Men run." Nonetheless, "men" in this sentence is a noun phrase.

In your example, "How the factors interact and their compound impact are not well understood", both "how the factors interact" and "their compound impact" are noun phrases, so they can be conjoined to form a noun phrase, which can then be a subject. No problem. The fact that these two noun phrases have different derivations is not relevant to whether they can be conjoined.