Yes you do need to either add other devices manually or by importing a list... but it's a one-time thing so it's a non-issue IMHO.
Not bad for tiny environments. But at any scale, how does that handle machines being added in the future? You have to manually add every new machine after every change to WAC?
I've only had about 100 servers and a handful of Win10 in it, so I'm not familiar with handling that at scale. It really only requires a txt file of a list of devices which is simple to maintain in any aspect. I only had to import that to get everything back in. If I added a new server, I would add it to the text file then add that server.