The method for sending pubsub messages from your control server to the extension users is documented here: https://dev.twitch.tv/docs/extensions/reference#send-extension-pubsub-message You can send whatever payload you want as long as it’s < 5KB, 1 message per second.
If you need to send messages from the broadcaster to the viewers using the live/config panels, you can use the send function in the frontend JS helper: https://dev.twitch.tv/docs/extensions/reference#send-functiontarget-contenttype-message
If you need to send messages from viewers to the backend, broadcaster, or other viewers, you cannot do this via pubsub. You will need to use ajax/sockets to send message to your control server, then have the control server broadcast the message via pubsub.
PubSub has been reliable for both the extensions I’ve used it in. I’m definitely not pushing any of the rate limits, but I haven’t recorded any outages or failed transmissions. It’s always live.