SEO

Probably?

The plugin blocks any visitor (search engine crawlers included) from viewing the website until they log in. Considering search engines won’t be able to log in and access the website while this plugin is active, I would guess this will have an effect on your search engine optimization (SEO).

If you’re looking to block the general public from viewing your site, I’m not sure why you would want to also have it optimized for public search engines.