Boxing in and of itself is not necessarily inferior, no art is inferior to another. However in the context of full contact martial arts competition, such as in MMA, there are several fundamental disadvantages to boxing.
1. The rules of the sport's standup department are built basically around the rules of kickboxing.
2. In most cases, unless its boxing oriented for MMA, the shins and legs are not conditioned in any way to withstand leg kicks. Plenty of boxers in the early days of the UFC were obliterated because they didn't know how, or possessed the capability to check leg kicks, or the muscle memory to evade them.
3. Boxing, and in some cases kick boxing, rely on the old fashioned "tank the strike" defense. This works with 16oz gloves, but with 4-7oz MMA gloves. That strategy ain't gonna cut the mustard.
All in all, boxing is far less applicable for MMA in my personal opinion. But there have been great boxers who've compensated for any kind of disadvantages and managed to be good fighters.
Sig courtesy of that photo-matic magician limba
I went out the way that I fight. I went out on my shield. That’s it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit