Sustainability

The most important thing
"Sustainability is a characteristic of a process or state that can be maintained at a certain level indefinitely." (wikipedia)

In PTC terms, when a site is sustainable, that means this program can live and pay even and for years. While unsustainable programs will pay in a short term only, from a few days to a few months.
Grossly unsustainable sites are scams.
Sites paying astronomical amounts (example: $100 per click), even if are charging more, are scams too: "If this were real, nobody would have a day job and 90% of the world's population would read emails for a living. Which does not happen!"

Very sustainable sites will pay you less than $0.0025 per click. Sites paying more than $0.005 per std click with more than 50% ref bonus are very unsustainable no matter what the advertising rates are or who the owner is. Such sites usually will survive for less than six to nine months, or will turn to bux.to schemes (very long payment delays, cash out limitations, ref limitations, bans without reason etc).
A $1 per click (for example) PTC sounds very tempting. But it is not something more than a big scam. If you don't believe it, find a program like this, register but DON'T invest and after a few days/weeks you'll realize the truth.