Nightly is our testing and development version of Brave. The releases are updated every night and may contain bugs that can result in data loss.
It might crash often or not behave as expected as its has unfinished features.
In addition, Brave Rewards features are also not considered stable in Nightly channel. Brave does not guarantee that tokens accumulated or granted in Nightly will be transferable to other Brave channels. There is a chance that instability and/or experimentation may result in the loss of tokens in Nightly.