Everyone responds differently to different substances. Even the same person in different times.
HPPD is definitly linked to this one, that's one of the main things I've heard. I've also heard of people who just couldn't handle it, and people who became violently ill(projectile vomiting).
Another problem with RC's is, mislabeled batches(some have actually died from this).
Shulgin is a great researcher! Though there's definitely some compounds he's found/made that I wouldn't touch with a ten foot pole personally.