99 Pervent of all hyips is scam so we have 1% leeway to get more money.when you invest on hyips you must know the time that you need to withdraw, invest again, or run.
Some HYIP's are actually run by honest admins and sometimes fail because of bad management. Others can fail by external events that affect them like hacker attacks or something like that.

What I will say is that they are all very high risk. You are basically trusting your money to an admin you know nothing about, and have no control after you give it to him/her. Even if it is an honest admin with the best intentions this is very risky, since you don't know about this person's ability to manage money, let alone a big business like an HYIP