Even the Raspberry pi has a few areas of concern with propriety firmware and closed codecs. It is not a completely open hardware platform, it does run code that no eyes can see.
I don't entirely get the notion that computing has to be 100% open for anybody to learn anything. My pulling-back-the-curtain moment was booting Linux on an Xbox & hand building a VGA adapter. Had never touched Linux or a soldering iron before that. Was it easy, no, but that's literally what made it so engaging & educational.