Still many SEO's think keyword stuffing will work or they think they are doing the right way without checking the site keyword density and it can bring those organic results. This is again one form of black hat SEO. Here is the Google guideline on "Keyword stuffing".
Inlinks bring popularity to your websites and link juice gets your listing top. Natural links are good and you can get those links by attracting your readers with quality contents. But many do go for paid links and it results in violation of guidelines. Here the guidelines on "Paid links".
SEO is a process that should be taken care by following guidelines. If you don’t do so then you have high chance to get penalized. As well have your content unique and quality. Do not have copy content, modified / rephrased content or content scraper that can lead your site to get penalized.