Even reasonable needs typically only go straight to an ultimatum when the person giving it understands on some level that it's not something the other person will give easily or happily.
For example my ex decided polyamory wasn't working for her, but I thrived in it in a way I hadn't in monogamy. I can't imagine she didn't understand on some level that it was an irreconcilable difference.
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.
The answer to most ultimatums is no. It's one thing if it's something you need to change and is reasonable to demand, but even then by the time it hits an ultimatum the relationship is often in a position where the damage runs deep.
I've ended an engagement over an ultimatum and it's one of the best decisions I've made
The build quality and reputation ruin it for me