Note that VST3 doesn't implement the COM vtable layout, their COM-like FUnknown really is just 3 virtual methods and a bunch of GUIDs. They rely on the platform's C++ ABI not breaking.
You're right that QueryInterface can return a different object, but that doesn't make it significantly more complicated, assuming you're not managing the ref-counts manually.
You're right that QueryInterface can return a different object, but that doesn't make it significantly more complicated, assuming you're not managing the ref-counts manually.