HuCards are region-locked, meaning that a PC-Engine game will not work on a TurboGrafx-16 (or vice versa) without modification. This is because the order of the data lines is inverted on the American version. However, as you can guess, this isn't something a cheap adapter can't tackle. That said, games may include routines to verify the system's region.
In contrast, CD-ROM games are neither region-locked nor copy-protected, but they still require a System Card to bootstrap, and the card itself is region-locked.