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"*.

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but to make it easily understandable and not jump deeper into advanced technical details - as [***skarz***](https://tor.stackexchange.com/users/22285/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](https://tor.stackexchange.com/a/228/5234) to a more concrete question.