#1 - In foreign niches, PBNs are THE BEST way to rank right now because the algo's are so outdated in terms of content quality & link signals etc but they are still good at stopping pure spam.. So PBNs are the perfect answer because you can use expired domains on foreign SERPs (I'd stick to non-dropped domains for English niches, more on that later) with literally foreign TLDs and as long as the power and content signals are there they work a charm.
Go into every Nordic (Sweden, Netherlands, Denmark etc) payday loans, casino or just about any spammy SERP and everyone in the top 10 is using PBNs.. You literally wouldn't be able to outrank them without having some sort of insane budget and tiered link building using PBNs on the tier 2 ANYWAY.
Danish payday loans site ranking #1 with a shit ton of foreign PBNs, not blocking bots so you can ahrefs it - penge247.dk
Germany casino site dominating for years at this point, using German homepage PBNs with branded anchors and good OnPage - onlinecasino.de
#2 - What niche/industry are you in, do you want to keep the site and how much money do you have?
End of the day, PBNs are cheaper than guest post link building.. I can buy an aged, DA/DR30+, RD100+ domain with most of it's links going to the homepage for $100 - $200.. That 1 domain gives me 100% control, 100% link juice and I can link to multiple other sites, or even sell links on it to make my money back.. Most niches that you reach out to will want you to spend $20 - $50 just for the link + the content + the time and you aren't getting 100% of the juice at all.
If you're in an affiliate niche and the #1 guy is ranking with a 50 site, dedicated PBN and you want to go full white hat, natural links, HQ content then good fucking luck with that one mate.
#3 - Sale valuations..
This one has always perplexed me.. Why people think they still need the PBNs when they want to sell the site is beyond me.
This is something I spoke about on LionZeal's show 3 years ago now ( https://www.youtube.com/watch?v=5cHoeMAGxa0 ) but the gist of it is you build and rank a site affordably with PBNs and as the site generates revenue you slowly pull the PBN links down and replace them with white hat links - You might see some initial drops at first, but link ghosting and a bunch of other factors considering you're already ranking play into effect and it's not that big of a loss at all if managed well.
I did this recently.. I had a site I made, used my own PBN on, ranked it with about $2,000 worth of PBN investment to a site that makes $4k/Mo on a good month and $2k/Mo on a bad month.. Then used the revenue generated over the next 6 months to flesh out the site with more content and remove the PBNs and slowly drip HQ guest posts - I sold that site for $72k.
#4 - How safe is it?
Is your niche prone to manual reviews? Where are you sourcing your domains? How fast do you need them? Can you do link swaps with other trusted SEOs?
Then there's a few obvious ones.. I build everything in FireFox (Just because I am that paranoid) use unique, premium hosting for each PBN, use the same writer for my money site content as PBN content (Just for future proofing any really good content algo's coming out) and slowly build PBNs up/out, as well as use wayback to see what the site had previously - Remember Google's cache is 2 years old, so it's likely they will compare cached versions, if there is any.