I found the online tests to be very accurate. The tests that undervalued your score existed about 1-2 years ago, and they are not really used anymore (unless you have one of their old CD's floating around).
The new tests are very good at predicting what your actual score would look like.
Also, don't put too much thought into Easy, Medium, Hard. They really have no correlation to what YOU think is hard. I believe they are based on 1000s of students and how they did at that particular problem. Then those problems were ranked by how often people got them correctly. A problem may seem easy or hard to you, but it doesn't show how the general population would feel about that same question. For instance, geometry, probability, and combinations are generally considered "hard" problems, and this would even be the same on the real GMAT. But in my experience, those were my best subjects and quite easy. I had to spend a lot of time on the "easy" questinos in order to do well enough on the "easy" material to start getting "harder" questions.
Hope this helps you out