Are HR universally incompetent? I recently rejected an offer from a company mostly because of their incompetent HR.

I don’t know but sounds like a bad reason to reject an offer. You probably won’t have much contact with HR in your daily work.

I had an offer from a rather prominent company, and when I had accepted and was finalizing the start date, HR tried to take the salary down 1.0%... and this is after all the negotiations & acceptance and hands were shook with the hiring manager.

1% is nothing serious (back then about 1,500/yr), but when the company pulls that kind of garbage, what else will they do later?

If I were not so shocked, I would have used that to negotiate up 5%. Instead, I was stupid and let my feelings get in the way. When I told her that the deal was already confirmed, she said, “Well, we are (Bigshot Company Name Here)..." as if they could do whatever they wanted. I politely said, “Oh, I see, then I will have to decline. Thank you very much for your time." and that was that.

Who’s competent these days ?