My biggest issue with the UI is that you don't have to actually click the radio button to change the mode. Also considering the fact that his config can and will cause your network to go down, I don't think it's very good UI design. Also you can click next to the text as well. I do agree that occasion for this is kinda rare and a person configuring should be aware of the shortcomings of the UI, but it doesn't mean it couldn't or shouldn't be better.
This is an option that should not be configured often. Usually it's an option that needs to be configured once during the lifetime of the MX. Currently this mode config dominates the addressing & vlans config page even though you never shouldn't touch it after the initial configuration.
By very default, I'd prefer if this option had a double confirmation like mentioned above.