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?
It's not very legit to use banner from Adsense in faucets. This is what Google Adsense policy say:
"In order to ensure a good experience for users and advertisers, publishers participating in the AdSense program may not:
Compensate users for viewing ads or performing searches, or promise compensation to a third party for such behavior.
Encourage users to click the Google ads using phrases such as "click the ads", "support us", "visit these links" or other similar language.
Direct user attention to the ads using arrows or other graphical gimmicks.
Format ads so that they become indistinguishable from other content on that page.
Format site content so that it is difficult to distinguish it from ads."
But if faucets still using it is because google permit them to do it