Yup! Mostly legal. We maintain a legal guidance page[1] covering what you can and can't do with the data. I assume our lawyers have been involved, but what do I know?
It depends. The "attribution" requirements can be a bit onerous at times, in ways that might put the most casual and opportunistic reusers of even CC-BY-SA data at some legal risk. (It's of course easier for a legitimate reuser with an actual stake in the data to dot the i's and cross the t's.)