Pretty much anything
What sort of limits do you mean? Rate limits? Or what can/can’t an extension do.
At the end of the day a Twitch Extension is just a static website. That can load data from an API.
Some content restrictions are covered here Extensions | Twitch Developers
Guidelines and policies https://dev.twitch.tv/docs/extensions/guidelines-and-policies
Technical reading Required Technical Background | Twitch Developers