In fact, Google even shares an example in its link scheme guide:
Toxic Backlinks - Example Google link in comments
Not only do these links have no value, they also leave a footprint that Google's algorithms can (and will) crawl due to the use of the same comments or very similar variations of them, all pointing back to the same page.
Avoid this tactic at all costs and remove any historical links generated this way.
Low quality directories
Let's get one thing straight: directory links sometimes get a bad rap, period.
And this is very much a "it depends" situation.
If you are listed on real directories that offer real value to users, you are not at risk, and often these links can be really good, as long as they are topically related to your business.
This means that the directory is niche-specific or localized to your local area.
The following example is from the New York Chamber of Commerce.
It's a membership directory, and you're not going to be penalized or get toxic links for being listed on a site like this:
But compare it to the image below:
Toxic Backlinks - Low quality directory links
This is a low-quality directory that has absolutely no value to users.
In fact, browsing the site, it is clear that it is not targeted at a specific niche or location:
Toxic Backlinks - Browse Directory Website
Avoid generating low-quality links like this one.
They offer no value and could do you much more harm than good.
That said, if a directory offers value in terms of inclusion alongside others in your niche or a local area, it may be worth considering, but not as a tactic to generate a significant number of links.
You have to consider each case individually because you are interested in more than just getting a link.
Guest articles on scale
To be clear, guest posting is not necessarily a link-spamming tactic.
In fact, it is often far from that.
However, SEOs started publishing guest posts on a massive scale after the Penguin algorithm was launched and it was often low-quality content anywhere that would accept them, even if the sites were completely unrelated.