No company can guarantee results. Ever.
The fact is that while SEO has a proven track record of success, the final determining factor in rankings is how search engines such as Google, Yahoo, and Bing valuate your website, brand, and SEO.
Any company guaranteeing results should be a red flag. Particularly because a number of companies promise and deliver immediate results, but more often than not they are using methods that violate search engine guidelines that can not only see those results disappear, but lead to potential suspensions and bans