I've an question, I've seen a lot of cryptocurrenty faucets using Adsense, but is it legal at all to put Adsense in that kind of sites?
Yes it legal and Google adsense have no problem with that .
However there is something you should know about their rules , like for example Gambling you are not allowed to put Gambling content on the website , also you can not make your users click ads to do something. (You can ask them to remove Adblock however you can't make them click the ads to go to another page or to get btc from faucet or whatever) you can get banned if someone reports you .