I think there is a HUGE difference between PBMining.com and Cryptcominer.com.

With PBMining.com, I've run some scenarios in a spreadsheet that show their business can be sustainable, i.e. it is not necessarily a ponzi.  I think what makes people suspicious is that they think there is no way that the current contract price can pay for 5 years of electricity.  I would say they are correct about that, however, the hardware gets replaced with more efficient hardware over time.  Then you probably ask where they get the money for the replacement hardware.  I think the contract price is high enough to cover that as well.  Many people can only visualize today's reality.  A year from now, the cost of purchasing and maintaining 1 GH will be significantly lower.  Nobody ever mentions that the expected return during the second year might be as little as 1% of the contract price.  With a 1TH contract, there may not be enough earnings to send a weekly payout after the second year.  If the difficulty rises too fast, then you might not break even.

With Cryptcominer.com, I just don't see how their business is sustainable in any way.  With this type of investment, there should be a prospectus to review.  The website is very vague, focusing on the high returns and the commissions that you can earn.  Whenever you invest in something that pays more than the risk-free interest rate, then there is risk.  If higher interest is paid, then the risk is higher.  20% a year is already high risk, so 21% a week could be 52 times more risky.  How long have they been in business?  It looks like one WEEK!  They say they guarantee the returns, but what is the real worthiness of a guarantee from a company in business for just a week?

In any case, PBMining.com and Cryptcominer.com both carry different investment risks.  If you're going to invest with either company, only invest what you're willing to lose.