I've written before that I don't want a mouse where the middle button is a scroll wheel. At the time I was brief about why, but today I'm going to expand on that and explain why you can't combine the two functions in something where both work as well as they would independently. In short: if you have a scroll wheel that is also a mouse button, using one or the other necessarily has to kind of suck.

The core problem is one of activation pressure. A good mouse button takes relatively little finger pressure to activate. You don't want too little finger pressure because then it gets activated accidentally, but the more pressure it takes the more fatiguing it is to use the button (and to some extent the slower and more awkward to do so). However, a low activation pressure for the button function of a scroll wheel is in strong conflict with using the scroll wheel as a scroll wheel.

To use a scroll wheel comfortably, you need to be able to rest your finger on the wheel without having anything happen. This means that scrolling itself needs to require some pressure to do, especially if you want to scroll in perceptible discreet steps (perceptible steps imply that there is extra resistance between steps, so that it easy to stop after a step). This pressure is not exerted directly downwards but it does involve a certain amount of downforce, especially if the user is a bit sloppy or hurried. To make using the scroll wheel work well, you need this downforce from using the scroll wheel to not activate the button function; otherwise, using the scroll wheel to scroll would come with bonus random button clicks, which is sure to irritate you.

Ergo you have a conflict. You can either have a nice to use button with a hair trigger, hard to use scroll wheel, or you can have a scroll wheel that works well with a button function that takes significant extra force to use. For relatively obvious reasons, every 'scroll wheel middle button' mouse I've seen takes the second approach.

(If you have a scroll wheel mouse, you can try this yourself; feel out the amount of force that's required to activate each button. With every scroll wheel mouse I've ever used, the left and right buttons have relatively light force while the middle scroll wheel requires perceptibly more.)

I kind of like the scroll wheel, but I use the middle mouse button all the time, almost literally. This makes the common scroll wheel tradeoff a very bad thing for me; it is exactly reversed from my usage pattern.

(There are also issues with finger positioning, where the scroll wheel is too far back for the tip of my finger to naturally rest on top of it. This might just be me and it might go away if I used scroll wheels more, which may come to pass.)