Google's core algorithm (RankBrain) is increasingly capable of recognizing spam, obviously-commercial guest posts & paid links.
And it seems to reward quality significantly more than quantity. In 2018, a single real, earned backlink is worth 100+ low-quality "easy links".
This was definitely not always the case. While the risk of getting Penguin-slapped or manual-actioned was very real, it was still entirely possible to rank (quickly) with stuff like PBN's, footer links, and volumes of Web 2.0 links (reinforced by thousands of scrapebox links on the 2nd & 3rd tiers).
Those days are over.
Effective backlink strategies that actually work, today:
* Landing high-profile guest columns / op-eds on industry authority sites
* Leveraging networks like HARO to provide industry perspectives to journalists in exchange for a mention, ideally a dofollow mention
* Helping webmasters fix broken links. With your content as a viable alternative
* Publicitiy stunts & actual newsworthy events
* Doing contests / sponsoring stuff in your niche, earning links (and press) in the process.
And so on.
The most scalable of those strategies is guest content.
Hope this helps
-Chris