Other PTC sites Scam Tactics.

Payment delays
Some sites in their TOS have this term:
Your payment will be processed and issued via AlertPay/Paypal within 30 business days for premium members and 60 business days for standard members.
30/60 business days are approximately 42/84 calendar. 84 days = almost 3 months.
Extra10.com, Earn3.com and Taketheglobe.com with the waiting line feature, proved that this term is a sign of an unsustainable site. The truth is that owner can pay only 2-3 members per day, max 10 members (reason: lack of funds), that's why these long delays.
Sites with such long payment delays are not sustainable and will fail soon.

Upgrade - Ref prices
Sites that have very low prices cannot afford the higher rates/ref clicks. If a site charges less than $30-35 for one year of premium membership or less than $0.80-$0.90 per referral (purchase) is unsustainable. But this does not mean, that a site with "normal" prices cannot be a scam.