By the past, I have used several free wordpress themes from the free theme repository from WordPress and I gave up. The issues are always small but when you put them together, it ends up in a big difference with paid themes.
No theme options panel
The paid themes come with a theme options panel which
Issue with layouts
Most of the time, free wordpress themes don’t have alternative layouts like
Problems with SEO
I already installed free wordpress themes and found HTML errors in google search console because of duplicate titles from archives or categories
Media are indexed
In wordpress, pictures can be embedded in pages called media pages. Sometimes those pages are indexed by Google. However, those pages don’t bring any information to the viewers.
Designed without SEO in mind
Many free themes have too many links on it. It means that there are lots of google juice leakages on the pages.
This is a major problem. When you use a theme that is very popular like Avada, Enfold or Genesis, there is always an answer to your questions on a forum or on a blog. When you install a free theme and if you want to customize it, you have to find out how it works by investigating the CSS or PHP files. Sometimes, it is very complicated.