Consider: The risk for React is based on 1) speculation that Facebook has patents on core React concepts and 2) the fact that patent grant that comes with React terminates in some situations if you end up in conflict with Facebook. So for safety, it might be better to make sure you use code that Facebook has no patents on, which (if (1) above is true) means "code that is nothing like react at all".
A React clone that doesn't have the patent grant at all is vulnerable to all the same issues React itself is, plus more. It's strictly more dangerous than using React itself.
(Unless, of course, we assume there's some bit of React that's subject to a Facebook-owned patent that would not also apply to Preact. But having spent a few minutes thinking about it, nothing leaps out at me.)