Not really. In embedded environments you're constrained by toolchain and platform but it's still a bad idea to rely on any behaviour which your compiler doesn'

Not really. In embedded environments you're constrained by toolchain and platform but it's still a bad idea to rely on any behaviour which your compiler doesn't provide a definition for (which might be more behaviour than what your standard provides a definition for) because changes to the version of the compiler or even changes to surrounding code can trigger issues caused by reliance on UB. It's not actually that hard to write embedded code which does not invoke UB outside of register access and even there it's possible to limit yourself to invoking behaviours which the combination of hardware + compiler does provide documented behaviour for. (source: I've written embedded code which did not knowingly/intentionally invoke UB outside of register access and in those cases the implementation did define behaviour.)