not sure because Canada does the master's and CFY, so not sure how the expectations/requirements for the CFY would differ from that of the externship.
another thing you have to consider is certification and licensure. certification is not required but state licensure is required in most states.
one of the requirements for state licensure is usually either certification or 1820 supervised practicum hours with a state licensed audiologist.
also there are the requirements for certification. ASHA requires 1820 supervised practicum hours by ASHA- certified audiologists only. ABA requires supervised practicum hours with a state licensed audiologist.
so basically applying for licensure in the US could be tricky if you do an externship in Canada. there might be some way around this but i veered on the safe side and decided to stay in the US for a 4th year. a former student in my program went to new zealand for a 4th year, but his supervisor was ASHA certified which ensured he would be able to apply for certification and licensure afterwards.