well nobody will be able to tell you "How safe are they exactly", especially as long as you don't describe exactly what you mean with "see a user's traffic".
but to make it easily understandable and not jump deeper into advanced technical details - as skarz partly mentioned, just using HTTPS and HSTS will do the trick. so usually exit nodes will only see, that some Tor-user is surfing on site "www.domain.tld", but nothing else.
i'm pretty sure this site is very interesting for you and playing a little bit around with it will make things clear:
https://www.eff.org/pages/tor-and-https
if you are still interested into a little bit more details, this is a still not techy more detailed answer to a more concrete question.