How about things like opening a file, where there could be a variety of reasons it could fail?
You should use a union type for that situation. Note, the current File API in the Pony standard library doesn't do that and it's something that we intend to fix before version 1.0. At this point, now one has gotten frustrated enough with it to open an RFC to propose an updated API. Someone will at some point though. But... volunteer project, limited time, itch to scratch etc.