Learn English – How to use ‘There being”

You may certainly use a clause which acts as an NP as the complement of in terms of. Here's the first Google hit on "in terms of how hard":

One of the major concerns of theoretical computer science is the classification of problems in terms of how hard they are. - Gasarch, Bounded Queries in Recursion Theory

But the particular clause you employ is problematic for me: how does the actors' difficulty in rehearsal factor as a term in a spectator’s enjoyment of the performance? (That, however, does not appear to be a problem which we can address here.)

FumbleFingers’ comment observes, cogently, that the problem may be the use of in terms of itself. Understood literally, in terms of announces the terms or categories you will use in speaking of the matter at hand; more generally, it is ordinarily used to head a phrase/clause which restricts the action of its main clause to a particular aspect or perspective which you are taking under consideration. You might write

This performance represents an extraordinary achievement in terms of how hard it was for the actors to prepare for it.

There you are judging the achievement in terms of—‘under the category of’—its difficulty, and excluding (for instance) the aesthetic effect and the emotional impact.

I don’t think that’s really what you mean; I think it more likely you intend something like

This is a great show, all the more impressive if you know how hard the actors had to work to prepare for it.

He did not complete the race due to exhaustion. He did not complete the race due to his exhaustion.

As the other answers allude to ("due to illness" = someone's illness; "due to her illness" = she was sick), "due to exhaustion" would probably be the runner's exhaustion, but might be someone else's. For precision, you'd want to keep "his exhaustion" in that example.