NIH and Harvard - It's About Values
We should expect that under the NIH and FAS policies, some faculty will chafe when they can't just sign the publisher's form and have have to start using a contractual addendum or some other legal notice in response. They'll become even more exasperated if publishers respond by amending the amendments. As masters within their respective domains, faculty are likely to feel some frustration reflecting discomfort with the unfamiliar, and perhaps alienating, legal language in both the publisher's form and the addendum.
The key point is that this really is not a technical conversation. It's a conversation about values. Contractual language is designed to capture a shared understanding among the parties, and the likely back-and-forth about copyright reflects a difference of opinion about the value of public access to scholarly thought and research. The point of the back-and-forth over copyright is to persuade the publisher to accept NIH's and FAS commitment to equitable access to scholarly research or to force the publisher to reveal that it operates under a different set of values.
So while it may be tedious or uncomfortable for a faculty author to have to carefully read the language of any agreement or addendum, that author must do so to effectively express the shared values of the Faculty of Arts & Science in Harvard's case and the American people's in the NIH case.