How big H1 heading is for on-page SEO?

Guys, what is your personal opinion and experience on H1 on the page?

I was taught that it's huge and there always should be only one H1 one the page, it should contain main Keyword, be short and understandable and relevant to the content.

When I'm auditing websites I always pay attention to it. But sometimes it appears that websites with few H1 or missing H1 have good rankings.

What are your thoughts?