The Google Assistant can control smart home devices, set calendar reminders, and play almost any piece of music in the world. It's also good at unit conversions, and that happens to be something I use quite often. Not right now, though. Unit conversions are broken on Assistant across all devices.
Give it a shot yourself: ask Assistant "how many furlongs in a league," or "200 pounds in stone." Feel free to use more normal conversions, too. None of them will work on Assistant via your phone or Google Home devices. On your phone, you just get a search query response. On Google Home, Assistant tells you it can't help with that yet. Currency conversions appear to be unaffected.
Based on threads started on Reddit and the Google Product Forums, this issue started last night. Users of multiple languages have complained, so this is most likely a problem everywhere. Someone on the Google Product Forums says Google is aware of the issue, but there's no official word yet. Let's hope for a quick resolution.
All is well in Assistant land. Google fixed this bug and conversions are working again.