in the riding the dragon ceremony, you are taken on a spiritual journey by a dragon. but this is all astral, not physical. astral dragons can do harm to your astral self, unless you summoned them. i've never summoned one, but they can do harm to you, the same way a spirit or demon can. in the astral though, your physical self will be fine. astral dragons aid to your spell work, and will help you with knowledge, bus also your spirit. the riding the dragon ceremony is pretty intense, some people don't get past it. so in terms of physical harm, you're in nor danger, it's mostly astral and mental if you upset them, or you can't handle your past.
astral dragons are just like the dragons you read about, they can be European, Ling, Wyvern, Lindworm, so on. some breath fire, some breath ice, some have a wicket bite, some fly, some can't, it comes down to the breed. point is, if they attack you, it'll be a different outcome depending on the dragon. if you've summoned them, they can do more damage, they could attack your physical self. i've heard a couple stories about dragnos attacking people, scratches, some bite and bruises, also heard about a couple small house fires [depending on what dragon you summon it'll change the temperature in the room, so i sort of believe it]
normally, dragons are there to guide us, if you have no fear of them, and are respectful, you can learn a lot. also, they speak in riddles and symbols, so get use to that. learn about runes along with dragons, and a couple riddles won't hurt. i've been dealing with them for years, and have only been attacked once. what happened was i was told when i first started to work on my chakra's, eventually i got lazy and stopped working on them and everything went back to how it was when i first started out. so i went to ask the dragons for help, and a fire dragon stabbed me in my chakra points. wasn't fun, didn't project for a while after that. my point is, they can be helpful, but if you upset them look out.