You do have a lot of fair points I've been working on the README recently and will work on it a bit more soon, most of my work has gone into the actual code. Currently it's intended for extensions that:

1. Apply to only a few specific sites 2. Their functionality isn't entirely browser API based (this wouldn't make sense even if they're polyfilled) 3. "Could be" userscripts: It's an abstraction layer to make things work, not something to build on top of.

That aside it's pretty powerful and does a really good job on most extensions that meet this criteria. I'll let you know once I'm finished with the README, I'd love to have some feedback on it. It's worth noting there's a draft (totally human written) blog article in docs/article.md