I totally believe they are legitimate, however I think that you're just very unlikely to make any money from using it. The numbers always look good, but due to the tick-tok of increasing difficulty it is a law of diminishing returns. More likely you'll almost but not quite make you money back compared to just taking the initial investment and putting it into whatever coin you are mining. If Bitcoin, Ethereum or whatever goes up in value in the mean time you'll feel like you made money - but you really should have just bought that coin in the first place and you'd have made more.
I looked into their Ethereum mining contracts a while back here: https://steemit.com/mining/@o1o1o1o/ethereum-mining-profitability
Basically if you can get mining hardware for free, or you can get power for free or very cheap (like Genesis do) you'll make some money mining. But you need a huge scale operation to make a lot of money.
So why do Genesis bother selling contracts in the first place? Well, the price they charge you is more than what the hardware and future power costs them. So every time they sell a contract they actually get big infusions of up-front capital and that way which they can use the extra money to buy hardware for themselves. So basically folks buying those contracts are giving them free money to finance their own operation.
Is it a scam? No, but it is misleading. Again - if you want to mine and make money I'd say beg, borrow, scrounge, or steal hardware and power and then off you go!
RE: I don't believe Genesis mining is a scam.