Obviously no court is enforcing the death penalty on 7 mitzvos bnei Noach today, but the fact that certain aveiros would be chayav misa should be a warning to us that they are more serious than we might have otherwise thought.
If there were courts enforcing the death penalty on 7 mitzvos BN, it would be in the context of a society that has as a whole accepted those mitzvos and understands their severity, so the application of misa would be something that happens rarely and after a lot of deliberation, hence the Robespierre analogy does not fit. But the fact that the death penalty does not exist today in practice for 7 mitzvos bnei Noach (except for murder in some jurisdictions) should not lead us to think that prohibitions are less severe.