I have a multisite using the core domain mapping function. I am also making use of the SSO feature from the hub. The sign on works quite well when connecting to the main site of the multisite but once you have to connect to a subsite, it requests you to login again. When I had a look I did not see an option to enable cross site login like the one in your domain mapping plugin. If I missed it, please do let me know.
As the SSO login is so convenient, it would be great if I could do the same with the subsites.