Occasionally there will be some coins that for one reason or another end up being profitable to mine on nicehash, or sometimes nicehash ends up being super profitable itself because of a coin having high demand. It requires heavy monitoring of exchanges so you know how much to try and mine and can dump onto an exchange and a still get your payments before any price drop. Nicehash works as an actual marked with bids and asks so its a lot more fairly priced, its also basically sold by the actual hash, so if you want to say submit 10,000 shares of a given coin you can order that much and either pay the market rate or wait and see if others are willing to sell at your price, either way you don't actually give up the money unless/until the order is done and it can be done in an hour or a day or whatever you like, but no one rents for an entire month at a time let alone 6 or more.

With these other "cloud miners" you are basically giving out a loan to someone with almost no recourse if they do not perform, in most cases its even specifically stated in whatever agreements they have that you are taking on all the risk, if the mining doesn't perform, tough luck, and they all have long term contracts that pretty much ensure you will stop earning anything before you make a profit. They will either do something like eobot did and say "oh sorry we can't afford the power bill any more want to have our old stuff now that its worthless?" or they will just take off with the money whenever the marketing gets over powered by people complaining about how they lost money and the fresh money stops coming in.

TLDR: Unless you REALLY know what you are doing you are better off just buying whatever coin you want to mine directly.