Even though global blocks are shown as a widget, the block to add them does nothing.
This is because the Widgets doesn't support "core/block" which is what is reported as an error.
If you register the core/block, it does show though but is broken once loaded in.
We are wanting to use this on a client project right now for A/B testing Ads for them.
Also would be good to be able to create the Global block from inside the Widgets screen instead of elsewhere.