I have a theory;
some people telling on forums they getting more organic seo when they switched from wordpress to squarespace.
Other people telling the opposite.
I think it has to do with schema markup, squarespace has this integrated in their cms (or maybe other cms too) , wordpress not. In SS you add schema markup easy in a sidebar for every post you write then fill out the schema fields that's all.
But SS not has good fast loading speed the SS team is using the fast keycdn on all SS sites of their customers so why is the site loading slow, keycdn is really fast I wonder. Today you can add free cloudflare cdn to your ss site it that is helpful and make the site faster? I do not know.
WP not has this feature you get plugins, but so far I not have found a schema plugin who shows me sidebar of schema fields on every post I write, you must fill out that manually, then insert that manually in your post html editor view.
You see the big sites who ranking at top, they all using schema markup, so I think that many wordpress users not doing this, then so google will not index all or most of your pages/ posts.
That is the only logic thought I have about this, another reason could be that the seo or sitemap plugins you use have many limitations (free version) so when you are over the limit the seo or sitemap plugin not work anymore. Maybe the plugin is outdated or has bad programmed code, this will harm you too.
My theory about this. Then when people here on BHW write that google only index 30% of their content , then for my logic this has nothing to to with the G sandbox.
I think the G sandbox not exist anymore today, when you buy a domain within three weeks your website is indexed in G.
Happy weekend