Potential problems with themes not in the repository of themes?

Hello,

I would like to use a theme which – unfortunately – cannot be found in the WP repository of themes (either Topbest https://demo.themesmarts.com/topbest/) or Celestino https://demo.yithemes.com/celestino/).

Would you kindly explain what potential problems I might run into? Are there specific issues why these two themes have not been reviewd and approved?

I could find no answer to this question anywhere in the forum. Please point to answer if I missed it somewhere. Thanks.

Kind regards,
Otto O’Mallee