Both. If you are using an iPhone, to record an hour of footage, it takes up a TON of your memory. Your phone might not have enough available.
Second, even if your youtube account is "verified" to limit the 15 minute time limit, many people have still reported issues actually uploading the videos and youtube accepting them. This came up in the Dr Dave thread. I personally, have tried many times, and have not been able to upload more than 4 or 5 minute from my iPhone to youtube.
In contradst, people using standalone video cameras (not iPhones), seem to have better success. There is no memory limit in the camera itself (at least nothin a Ghost session would exhaust), and youtube accepts the videos.