Is beta software is safe?

There is a risk. See the following from https://beta.apple.com/sp/betaprogram/faq

(bold added by me)

Please note that since the public beta software has not yet been commercially released by Apple, it may contain errors or inaccuracies and may not function as well as commercially released software. Be sure to back up your iPhone, iPad, or iPod touch and your Mac using Time Machine before installing beta software. Since Apple TV purchases and data are stored in the cloud, there’s no need to back up your Apple TV. Install the beta software only on non-production devices that are not business critical. We strongly recommend installing on a secondary system or device, or on a secondary partition on your Mac.