I understand that today link exchanges are amateur and are not as effective for SEO as they were a few years ago. Let's say I build a private blog network of 10 - 20 sites, all with unique content. Let's also say I use about 5-6 different themes across the sites (so some sites will have the same theme). Let's also say I use private registration and I use a different cloud server (with different IP addresses) per site for hosting. How effective would a private blog network be for SEO?