As far as cons are concerned, free themes have limited features and support compared to premium themes.
You will be missing some of the essential features such as SEO friendliness, fast loading, responsiveness, and more.
Another is there is no uniqueness in using free themes because free themes are accessible to everyone.
So you might spot a website that has the same theme as yours.
Furthermore, by using free themes you are inviting bugs and viruses that could completely ruin your site if downloaded from an unreliable source.