Using a free website builder isn’t always good for your brand because it's not customized. While some templates might provide a nice look for your website, it's not custom-fit to meet your brand aesthetics and brand values.
Free website builders show advertisements in your web pages which you have no control over. Note that some advertisements are very annoying to some customers which can lead to withdrawals.
You are limited to the free inclusions of the service. Almost always you would need a premium subscription to avail more.
Most of the time, website builders provide a free domain name. However, the domain they usually provide are subdomains of their company (e.g. johndoe.company.com). This makes you less credible because you are parking your own business to a different entity.
Sooner or later, when you decide to upgrade your website, you'll realize that it's not easy to move and migrate your content to a new platform. So, if you have plans to scale your business, choose wisely on whether to use a free website builder or hire a web design company.