I think it's typically true? The instances I know of where transactions were undone involved convincing all the node runners to fork their nodes - they were not

I think it's typically true? The instances I know of where transactions were undone involved convincing all the node runners to fork their nodes - they were not done "within" the blockchain system. Instead they used social consensus to create a slightly altered blockchain system. How big of a difference this makes to you is up for debate.