It seems idsite=all doesn't work for API.get maybe because of a problem with some plugins that provide a .get method.
As part of this issue we should add a test for all GET APIs with idsite=all to see if other ones break too. Depending how much work it be we'd then deprecate this feature or fix the issues. It's not clear yet if only API.get (and I think VisitsSummary.get) is a problem or other methods too.
If anyone experiences this problem in other API's too be great to comment here.
Generally some issues are expected if eg goals, segments, or dimensions are included that are only available for some sites.