CC0 can be difficult, how do you know it is CC0?
Just because Bob publishes it as CC0, do you know that Bob does actually hold the copyright on it?
What if it was actually John who owns the copyright, he can now sue Bob and you (it has happened to Udemy instructors).
I would avoid it if it was me, it is easy and cheap, but it can cost you $1000's.
Thor Pedersen - IT , Project Management, and Cyber Security trainer