It's not just VP though. When Reagan went on and off the deep end, it wasn't the VP who took over it was his wife who ran things. Who's going to keep things in order when Hillary is temporarily incapacitated Huma or Bill.
RE: Reagan; I think Reagan's position is a little different because there's no credible argument that he was suffering from Alzheimers in 1984. The most credible accusations that he faces that he was ill during his presidency were that the symptoms appeared in the back half of his second term. And most of the accusations seem to suggest the symptoms were obvious in hindsight but no formal disagnosis was made. I think there is a different standard of disclosure for prospective job applicants than for those who have a job. A better example would be Roosevelt's final term or Kennedy's term; both suffered debilitating health problems that they hid from the public while in office, and I think there's a strong case to argue both did deceive the public.
RE: the line of succession; I agree that modern bureaucracy is very complicated and relies on a trusted circle being able to speak on the boss' behalf. The presidency would not operate if chiefs of staff or assistants could not be delegated minor decisions that are on paper the president's to make. I also agree that during a time of crisis, this can result in unelected officials making judgment calls, which is part of the allegations made against the Bush in the wake of 9/11. It is important to have bureaucratic best practices about how this delegation works. It is important that the VP, apart from having the legal responsibility to take over, is assertive enough and well liked enough that they have a moral authority to take over in a tough situation. I don't want to talk about it being specifically Huma or Bill. She clearly trusts both, but we'll wait and see who she chooses as a Chief of Staff and who are the power players in her actual administration.
I am not sure that someone's inner circle is worthy of scrutiny from this angle before being elected. I do think their inner circle is worthy of scrutiny as a reflection of their judgment--Hillary's advisors should be critiqued as should Trump's be, but I don't think it serves us well to go from "Someone has a health issue -> What if their Personal Assistant is the secret unelected president??!?". The VP is the person legally entitled to serve in the event of incapacity, and both parties picked VPs that are well within the bounds of acceptable beliefs and reasonable experience for their party in a way that Palin was not at the time.