Yes it does, because getting accurate numbers from Facebook is a key part to calculating either of those things. You definitely can't do it with certainty, but you would hope the entity you're paying to record the data necessary to get as close to possible would in fact be an accurate source of said data.

If you are incapable of measuring the overall ROI of, say, a YouTube campaign vs. a Facebook campaign, then you are also certainly going to be incapable of determining the ROI of an X second average viewing time on Facebook vs. a Y second average viewing time on YouTube.

Therefore both the true and reported values of X are completely irrelevant because you are incapable of using them.

Please explain how the above reasoning is incorrect.

Detecting differences between several Facebook campaigns?

They would both be misrepresented the same way in that case and would remain comparable.