Actually, that's undefined behaviour. You are not allowed to read from a member that wasn't written to directly. Also, casting a pointer to unsigned char to a structure is also undefined behaviour, since there is no guarantee by C++ that such cast is valid (e.g. due to alignment).
It's not defined by C++. It is defined by the compiler. I've never come across a compiler that had a problem doing it correctly, even without pragmas. Endianness may be a concern if you port it, but that's easily solved. In short, if you're iterating through units and casting them as byte arrays then you're already in the 'grey area' even though it's something that has to be done all the time. May as well make it easier to work with.
I should probably have used uint8_t there, though, since I used uint32_t.