this is just basically cloud mining. You are buying a plan in order to make more money? they use the money to buy more machines and pay for rent, and then give you a small cut of what they mine... but each machine will slowly get worse and worse profit wise and soon they will be at a point where they must sell the miners and who knows what happens when they do.
you would make more money jsut investing the same money into the coin itself. Also, what happens when its a sunday and payout day is a friday and bitcoin is crashing hard... there is no way to pull funds.
There are too many problems to state here. Mainly they are up charging you for "hashing power".
100 day return on investment.. they mean this in the amount of BTC paid back. That doesn't mean BTC will be worth that more much after those 100 days. its possible it would be worth less, but you are stuck in a contract with them. Also, mining difficulty is constantly rising, and they wont be able to maintain those ratios unlesss the mining difficulty went down (wont), or if they are constantly spending users money to buy more and more machines, which again, will eventually not be worth much.
7nm and even 5nm chip sets are coming soon... there's no point of paying for hash that will just become less profitable over time.... I don't see why people do this unless they see a narrow window to make some quick cash on a starter pack or something.
I can't say the same for bitconnect, but I can tell you there is support for mining it, because well, I mine it, but the concept can't go forever IMO.
these websites also pay people to add more money to their accounts and get higher returns in the short term to show "how rich i got off (insert cloud mining opp here).