I think there's a difference between a hard boundary and an ultimatum. The former exists before it needs to be said, and the latter is a "do/stop this or the relationship is over". It's often associated with existing relationship issues or a sudden change in the relationship.

It's also associated with an expectation that one gets their way. Rather than approaching as a conversation about "are we still compatible" it becomes a cudgel. When I rejected my ex's ultimatum it resulted in her begging me to change my answer and later yelling at me about my being willing to end the relationship.