Ask HN: Is it normal for seed rounds to have milestones?

Is it normal for seed rounds not to be paid at once but in milestones? Should companies accept this?

The worry is that if funding is tied to milestones then if those milestones are missed we would have to go back to fundraising at the worst possible time - whereas if we shop around more we might find an investor who does not tie us to it.

Basically isn't it like a time bomb - that can blow up your startup, even if everything is going well. Why would startups accept this?