Square Space Struggles - Is it worth the bother?

[KarenJames]

Hi all. We have always struggled to rank Square Space and Wix sites. In one case we switched a site to WP and it ranked for a local term within weeks. Does anyone know the technical reason why SS & Wix seem to have this issue?  I have searched other questions and have read that SS sites ranking well are hard to find.

I have a potential client with a SS site that is on the floor and he does not want to part with it. I would like to be able to give him some specific information to persuade him to move it.  Thanks in advance.