The legit cloud minings are really mining crypto-currencies, different from the Ponzi HYIP schemes. But it doesn't make much difference, because it's no profitable anyway... A legit cloud mining doesn't promise you a lot of fast profit (different from Ponzies), they promise a small income for LIFETIME, looks really good. But when the days, weeks pass, we see that the small income is near to 0, so it doesn't worth.
It is very much like lending out a loan without setting the exact date by which the loan should be returned. And I don't mean so-called call loans which should be returned on the first notice, i.e. repayable on demand. A borrower may just promise to return the borrowed amount some day during his lifetime (when he becomes rich, for example) which basically means that the loan will never be repaid in the majority of cases...
In short, a fool and his money are soon parted