When using mutli-platform build, cancelling the operation early can cause the SDK to lock up. No buttons appear in the build panel, and everything will be greyed out as if it is waiting for the build to finish. This cannot be recovered from until the Unity editor is closed and reopened.

It's hard to say whether when I cancelled it was in the middle of it building or it swapping platforms, but the latter seems extremely likely point of failure as cancelling mid-build for a single platform has always worked normally.