The v5.2 release will be out on Friday. We're prepping everything for it's release now.
The v5.1.2 release doesn't have specific EasySocial integration. It does create standard Joomla users which EasySocial detects, but avatars, profiles, etc won't be imported into it.
The v5.2 release adds EasySocial profile integration so you can import profile information, avatars, and cover photos from Facebook, Google+ and Twitter into it. There's also a new registration option where you can use EasySocial (or JomSocial or Joomla) for the registration path. That way, you have to only manage one registration flow on your site, and all the user information will be pre-filled, or optionally hidden, making the registration process much easier. Automatic registration is still an option as well, for the social networks that support it. Twitter will always have to use a registration form of some sort as Twitter will not provide the user's Email address.
To what extent would Social Login/Register break under the current 5.1.2? I just want to understand so we can plan.
I'm honestly not sure what you mean here. Hopefully the above explained it, but if not, let me know.
Thanks,
Alex