Like any free webhosting service, you are given a subdomain, not your own domain. For example, mysite.yolasite.com rather than www.mysite.com. Since you're on a subdomain of a real domain name, your site's search engine ranking can be negatively affected by other subdomains on yolasite. The same thing happens with weebly, webs, 3owl, and other free hosting providers.
A free hosting provider can also change its terms at will, which means that it could place its own ads on your website, or it could bar you from placing your own (or both) even if they've been generous in the past.
Long-term it's better to invest in the domain name that you want to own and pay the monthly hosting fee with a separate paid-for hosting company.