The difference between regular SmartLipo and MPX is that the laser is a bit more powerful with the MPX. While that ought to be a good thing, the problem with this as well as any other laser technology is that too much of a good thing can be bad.
In other words, sometimes less is more.
When you are performing laser treatment of fat, the one thing you want to do is to melt the fat enough, but not so much that you might have burning or an unusual look to the skin that can look like a red mat of blood vessels. This has been the downfall of many previous treatments like ultrasonic liposuction and others.
While the original SmartLipo definitely has power, especially in the 18 watt version, the MPX may have too much power and therefore cause these problems. Additionally, the really good surgeons use tumescent liposuction along with it to get the results that are truly excellent.
If a surgeon is only relying upon the laser and not doing the tumescent method along with it, you probably won't have the results you want anyway. That's why MPX may not be worth the hype.