That could be most web apps functionalities. Things like registration, authorization/authentication, sending emails, store/retrieve data, etc...
> so using them to do “serious number crunching” at the edge, which is the advertised use case, seems questionable.
Cloudflare workers don't run in the background. They block the HTTP request. For serious computation, Cloudflare should offer background workers that can run for extended periods of time. [1]
1: This could be tricked by triggering an async request, but there is no push API to get notify the "App" of the result.