I understand that it makes sense to not write these types of user functions and management things over and over again. The solution however, is not a SaaS, but a library or a little framework. And from what I remember, major Web development frameworks offer exactly these types of functionality.
I don't want to be a downer and you guys probably spend a lot of time on the product, but from my perspective, any business owner using a third party to handle user data acts irresponsibly. You OWE it to your users to keep their data as tight and as centralized in one spot as possible – a spot only you and employees have access to and servers only you rented and have access to and not a third party.
I'd even rather use Wordpress as a basic user management platform than use a third party service. This way, it is at least fully under my control and I'm the only one responsible if things get broken or data gets stolen.