Whitepaper is a necessary document to have. All ICO need to have the whitepaper.
But it is not a guarantee!
Even if the ICO has the whitepaper, it can be a scam project. you should make your research before picking any project and do not invest into any project that does not have whitepaper
Yes, what is important is the authenticity of the project. The white paper alone can not be fully understood. It is necessary to assess the maneuverability of the project in all aspects.