I have had to replace a couple of smoke unit PCB boards when the smoke regulator runs away and burns the board, but I've never replaced a complete smoke unit yet. You can fix almost anything on a smoke unit, normally the wick and perhaps a resistor is all that's needed. Of course, there's the run of bad Lionel fans, but I've only replaced a few MTH smoke unit fans, they're normally pretty robust. Occasionally, a gasket is bad, I notice many Lionel smoke units have dispensed with the gasket nowadays. I suppose the new Lionel plastic smoke unit bodies might suffer in a thermal runaway, but that hasn't happened to me yet.
A smoke unit does require periodic maintenance, typically the wick. Proper filling and not running it dry will go a long way to a long life before you have to replace the wick. I'm also a big Megasteam user, my new favorite is Clove Sensation, and my wife likes that scent as well. 
