I'm generally positive about restricting undefined behavior at a lower level (syntax, undefined field accesses, etc), but at a higher level (writing large scale applications, using it outside of intended domains, etc) it's not very clear how and even if you should restrict unintended usage.

The more room you have for "undefined behavior" at a higher level, the more people can explore using a tool in various ways you never thought about. This can in turn be valuable feedback on possible directions to expand your tool in.