Some bad things are: Bad administrators- when you need help they won't help you good, or not at all.
No payout- They framed you, and they won't pay out your earnings.
Very low earnings- There are also sites which pay out not all they promise.
So you earn more but they don't put it on your account.
Or maybe they pay too late. Some sites can take months until they decide to payout.

There are a lot more of these bad things. Some sites also say you get 5$ each time you click a link, but when you cash out it was just used as a currency.
So you might have earned 10.000$ but it was only worth 0.50$. This happened with a site called onedollarptc.
So a lot of work but not as much money as was expected.

Some sites like free members, and also paid members. Free and paid is exactly the same. Except one thing:
A free member cant cash out his money. This is with usdcenter.

These are just a few examples, and there are a lot more of them.