The biggest problems with login box, a.k.a. drov.io:
1) Trust. Who are you?
2) Pricing. Even with 300 sites, 1,000 concurrent users at any given moment, and fully dynamic sites, the proposed pricing for me of EUR 70 per month is a quarter of all my current costs. The cloud has made things very cheap, and this looks very expensive as a result.
3) Multi-user and PaaS. How could I grant access to the membership and management of one slice of users but not the rest? i.e. my 300 sites are on one platform, each site is typically a non-profit who own their own users... in theory they own their user database and I just host it, they should be able to manage it and for me to query that to apply authorisation within my app.
4) Longevity. If I consider moving things over... what happens in 18 months time if you reach the end of your runway? How could I reverse adopting this?
5) How do I identify a user? Assuming you auth them and then say "you're good", what single thing can I use to identify them that is meaningful to me? Persona returns a verified email for all users, are you able to do the same?
6) Are you static site friendly? i.e. Can the pages with the "Sign-in" box be cached and thus allow me to serve great numbers with little hardware, and does loading a page with a "Sign-in" box always trigger a HTTP request to you (if I'm under DDoS attack, is it going to take you down?)
7) How do you manage "email change"? Or things like "I signed in with my Gmail, but now I want to use my Github"?
Those things weigh heavily. Happy to talk with you about them, I notice that you're in London and so am I. These sites are side projects, not my main gig... just things that linger and that I still maintain.