1. Great functionality. HREF Lang did everything I needed it to do, with far greater ease of implementation (and time requirement) than other similar plugins.
2. Great support. When I had an issue, the developer responded within a day, logged into my WP admin (with a temporary account I created for him), and pointed me into the direction of my error. Awesome!
I had been using Add Tag Manager to implement hreflang. There were two problems with that solution. 1) The ATM author added a feature intended to enhance hreflang that actually broke it. 2) ATM is no longer supported. Hreflang Manager is better solution for me because all I wanted from ATM was hreflang support, and that is what HM is specifically designed to do.