Multisite Registration Security

As I build out and try new sites on my multisite network I am starting to run into a problem, securing all the different login and registration methods of each site. the main site has both the default login page and a buddypress registration page. Another site is using bbpress. Another might use a different method. How do I keep all this login/registration activity in one place with one recaptcha or honeypot while also keeping users within their respective sites? If I redirect a subsite user to the default login, then they will end up on a different site from where they started. Buddypress forces registration through its own page, no option to redirect to a single login. My head is spinning, all these different ways to register and login seem counterproductive to how multisite works, a consolidated user base. With all the different login methods, I feel like security is going to be a big problem, the site feels like swiss cheese with lots of holes.

I guess my question is, what is the standard secure solution to working with multisite user accounts and subsites?