Home Depot Paint & Primer in One
 

I had high hopes for it. I thought that all the 'it can't possibly
work' postings weren't based on actual experience or knowledge, just
mistrust.

Well, I used it to paint red over a white threshold which was already
painted (years ago, not new paint).

It went on like primer- it stuck just fine but didn't cover/hide well.
So it needs a second coat to fully hide.

So I don't see the advantage, I might as well use regular primer and
then regular paint.