I've heard of the occasional problem, but no more than you may find with other entry-level priced equipment. I've had mine for about 6 years with no issues, and I know there are plenty of people on here who have had them longer.
For the price, MES smokers are pretty good. Not sure how much experience you have with smoking, but they are great to learn some basics of smoking and knowing what to look for. That's what I learned on anyways. I would recommend them without a doubt.
I have had my mea 40 for a year and a half ! And use it at least once a week or more ! Only problem I ever had is if the outside temp is below 32 I have to heat the sensor with a hair dryer ! Other then that it has been great !
MES smokers are not bad but they can be made better. The boxes themselves are actually quite good. There have been some that had controller failure but it seems Masterbuilt is pretty good about customer service (no firsthand experience here). Their smoke tray system sucks in my view - just a bad design. Won't make smoke at low temps and needs too much attention when it does make smoke. Many (most?) including me make modifications that turn them into excellent electric smokers.
I've had 3 MES units over 10 years, and all of them still work fine.
You have to realize that there are probably about 20 times as many MES being used on this forum than any other electric brands out there, so I would expect to see more problems reported from them, and most of them have been fixed and resumed Smoking Away!!!