Can you name any specific problems with it?
The fundamental fatal flaw of the design is that it is actually "vertical" which means that in order to use it you basically have to "pinch" it between your thumb and the rest of your hand. With your forearm twisted outward.
All manner of evil flows from this inherent problem.
Consider a "classic" mouse that lays flat on the table. Using even one finger, you can move it around in any direction, and by tapping it gently with one finger you can activate either button. The inertia of physics causes it to remain in place as a general rule, and this works to your advantage.
Now, just flop your hands out onto the table in front of you. What is their natural orientation? Vertical? No. Horizontal? No.
They are rotated about 45 degrees inwards.
So, by building a mouse that allows the main bulk of your hand to drape across it at about 45 degrees, with your thumb wrapping up and out in the opposite direction, you get the benefit of gravity and inertia stabilizing the mouse as you manipulate it, the strong massive part of the hand manipulating it, and the benefit of the opposable thumb acting as a counter force to the rest. (Gripping it exactly like the tree limb our ancient ancestors grasped eons ago.)
With the retarded Evoluent mouse, every time you click a mouse button, unless your thumb is working at exactly the same moment (pun not entirely intended) to counteract it, then the "equal and opposite reaction" is trying to move the mouse rather than activate the button!