I'm the guy that made Agar.io, Diep.io and a few smaller games. I analyzed the possibility of using WebRTC in my games several times so far, but it seems that right now, it's still hard to use in a server-client architecture. You need to bring this [1] behemoth and all of its dependencies to your project dependencies on the server side, even though you only care about a tiny bit of it (unreliable data channels). It's unlikely that people will start using it until there is an easy stripped-down version that only deals with data channels.
[1] https://chromium.googlesource.com/external/webrtc