Usabilitytesting.com recruits users from a pre-selected panel of users who have given consent to be a part of usability tests in general. When you test with these set of users, what you hope to catch are low-lying usability fruits like fonts, colors, labels, widgets, etc.

Key issues like navigation and intended behaviour nuances will not get addressed as the users are not 'intrinsically motivated' to complete the tasks. The users are neither motivated to apply for a Certificate of Deposit online nor interested in buying a pair of hiking shoes online for the upcoming K2 climb.

Usabilitytesing.com is great for things which most people regularly do online - buy pizza, check account balance, etc. However, it is not good to conduct in-lab usability testing when specialized people with intrinsic motivation for completing a task are required.