What's the problem with HARO, Qwoted & similar PR & link building networks?

I have seen a lot of agencies and clients boast about how they are able to make a lot of backlinks or brand mentions with HARO but seldom seen any individual boasting about it.
Though it has a lot of mentions in blog posts & the very old articles that rank on top in SEO.

Is there any unseen problem with networks like HARO, Qwoted, Terkel, Help a B2B writer etc that I am not able to understand.

From the way they are marketed, they seem like a really good way to build backlinks and one of the only thing that can make sense where you don't have to pay money or give any backlink, like the whitest way to build backlinks.

However, I have seldom seen any SEO or link builder boast about it on their linkedin page or their resume.

Is there something I am missing with this kind of link building or is it something that just doesn't yield the ROI because of things like low conversion, not enough opportunities or less backlinks and more brand mentions.

Would be great if I can understand more from someone who has already been there, done that.