Are backlinks harmful?

Avoid them at all costs.

Bulk building links in this way is the quickest way to get your site de-listed despite their claims. Bear in mind they want you to pay them money so they will tell you what you want to hear. 

As for what links are good, well nothing there has changed, users who link to you that are naturally gained are the best ones to get. Better ones are from high authority sites but they don't tend to link to other sites at your call. As an example, let's say the BBC did an article about a topic and posted an unprompted link to you. That would be the perfect link and gain you a lot of creditability. 

In addition to what Chedders as provided you....

Links with the 'No Follow' tag will not hurt your site.  You should be building backlihks for traffic and not ranking.  

Google now uses the tags, "no follow, sponsered, and ugc", as hints and may in fact count them.  The links will not impact your seo in a negative way.   I look at it this way, they are mentions and mentions do carry weight.  How much weight is up for conjecture.

As Chedders said, stay away from bought / paid links, they just cause trouble in the long run.

Low-quality or spammy backlinks can harm your website's SEO. It's not a good idea to buy thousands of backlinks from untrustworthy sources. Instead, you should focus on building high-quality backlinks naturally through creating valuable content and collaborating with other authoritative websites.

One tool that can help you build high-quality backlinks more efficiently and safely is Postifluence. This automatic backlink-building tool can save you time and ensure you're using ethical practices. With Postifluence, you can easily find relevant blogs that accept guest posts. Its automated system handles the outreach process, increasing the chances of securing strong backlinks and guest-posting opportunities. Give it a try!

Backlinks are not inherently harmful in SEO. In fact, they are a crucial aspect of off-page SEO and can significantly impact your website's search engine rankings. High-quality backlinks from reputable and relevant websites can signal to search engines that your site is trustworthy, authoritative, and valuable to users.

However, not all backlinks are beneficial. Low-quality or spammy backlinks from irrelevant or dubious sources can harm your site's SEO. These types of backlinks can trigger search engine penalties, leading to a drop in rankings or even removal from search engine results pages (SERPs).

Therefore, it's essential to focus on acquiring high-quality backlinks from authoritative and relevant websites through ethical and white-hat SEO practices. This involves creating valuable content that naturally attracts backlinks, engaging in outreach to relevant websites, and avoiding tactics like buying links or participating in link schemes.

In summary, while backlinks can be incredibly beneficial for SEO when acquired ethically, low-quality backlinks can have a detrimental impact on your site's search engine visibility. It's crucial to prioritize quality over quantity and focus on building a diverse and natural backlink profile to enhance your website's SEO performance.