PBNs work. But the best way is to build your own. As noted, the economics of it are increasingly difficult, but they do help with money keywords. I'm not doing any right now, but have within the past 1.5 years.
Consider this: $10 for expired domain, $12/yr. for cheap web host, plus you need to get up unique content, or do a website rebuild (cost $12).
$10+12+12 = $34 per PBN site, if you're doing it super-duper cheap. Better domains cost more, better hosting costs more. And if your VA or a service to build it for you, that costs more too.
Seems like most guys doing links at scale would rather pay $100+ for a guest post on a site with high DR and real organic search traffic. You're one and done, and the link is permanent. Better than building out thousands of PBNs, which require ongoing upkeep.
The only people that seem to still be relying on PBNs are greyhat SEOs in the local SEO space, because local SEO doesn't really warrant a huge content marketing campaign. Plus with many clients you only need 10-30 links to rank them #1 for their keywords. PBNs will make that easy.
But even then there are disadvantages to PBNs.
Personally I've never done PBNs at scale, but using them here and there they have helped quite a bit.
The key to not getting penalized is to not leave a "footprint". This means creating websites that look and feel different. (Different content, WP themes, different CMSes, website layouts, etc.) The hardest thing is scaling web hosting.
You have to get hosting for your sites on different servers. So keeping track of a bunch of sites on different hosts is a pain.
If all of your websites look and feel the same and are all on the same web host, that is how you get penalized.
Or are all on the same Google Search Console or Google Analytics account. Stuff like that that makes a pattern easy for Google to find.
PBNs these day resemble real sites to the point that some (not all) actually are. One such PBN provider I know actually decided to start a niche relevant PBN network. By the time he had 10 websites, all of them were ranking for search terms and had actually useful content.
That is not to say that 1 hour PBN builds don't pass juice anymore, but the guy has been building PBNs for years. If he doesn't do quick builds anymore, I'm not going to bother.
After you build a PBN you also have to toxicity test it. This takes about a month. Basically means to create a post and link to a site ranking on page two for a low competition KW. Then see what happens to the sites.
I'm not saying "don't bother", but you definitely need a lot of research. The amount of footprints alone is staggering; around 60-70 you need to be aware of.
For myself, I've decided to only ever use PBNs if I'm in a niche where they are ubiquitous. Otherwise, it's just not worth it for me.