Like other forms of marketing, SEO isn’t perfect. It’s considered better than most other alternatives, but it takes time for results to show. Some projects will take months for websites to pop up among the first search results.
Moreover, it’s an ongoing process, rather than a simple one-time operation with instant results. That’s because search engines change their algorithms on a regular basis, so SEO companies must adapt to figure things out.
Other than that, the effectiveness and results of SEO will also depend on the niche and industry. Some keywords (those with high competition) will be tougher to crack.