But doesn't that show why this is a bad idea? If I understand correctly, this code:

  const MyUnion = packed union {
    full: u16,
    bytes: [2]u8,
  };
  const value: u16 = 0x55aa;
  const in_union: MyUnion = @bitCast(value);
  const without_union: [2]u8 = @bitCast(value);
  std.debug.assert(without_union[0] == in_union.bytes[0]);
  std.debug.assert(without_union[1] == in_union.bytes[1]);

...will now succeed or fail depending on the endianness of the target. That looks like the type of footgun that will bring decades of joy.