It depends on where you put a banner. If you have a banner on faucet website, probably you waste your money, because they have unreal and shity traffic. But if you buy a banner to place on websites like coinmarketcap or similar most popular crypto related website, you have a big chance to get fine feedback audit from there.
Place where people going just to do one task are not too desirable for advertisement.
I concur. Basically, not all who sell banner advertisement are effective but rather you should be selective and be wise to choose among these. Also, in faucets, the exposure of your paid banner is small and people tend to not trust the dusplayed advertisement there. But if we take coinmarketcap for example, a lot of people stayed longer in this site and they have a mobile app which will maximize the exposure of you paid banner. Since it is trusted, people will have the courage to view since they know phising links are filtered out very well.