I really wonder what a broadcast domain looks like Pertino/Zero Tier/Himachi?
Depends on what kind of broadcasts. If you mean Ethernet broadcasts, which is what most people mean (the ones that are limited by VLANs) then these VPNs don't affect them at all.
So an ethernet broadcast storm could bring it down?
Ethernet broadcast storm can take down anything that is using Ethernet. What you are doing on top of Ethernet is irrelevant. But what do YOU mean by "bring it down?"