My experience is that Legacy smoke units need more adjustments/maintenance than MTH's to ensure a good volume of smoke and generally that there are more things that seem to adversely affect Legacy units.
I think part of the problem is that the Legacy units are designed to produce different smoke effects according to the operating characteristics of the engine. Mike Reagan once explained that there are 7 different voltage levels that can be supplied to a Legacy unit running under command control so it is not a simple matter of adjusting from low to medium to high to get the smoke volume you want; EFX settings matter too as does the reliability of the power supply to the layout.
Also, I noticed an immediate improvement in the output of my Legacy units when I switched from an MTH Z4000 to a ZW-L transformer so I deduce there really is something in the different sine wave output as described in an older thread here.
Lionel's definitely had more issues with fan motors than MTH and I'm finding that my Legacy units need regular cleaning of the motors because smoke fluid seems to "leak" or be blown on to them from the chamber where the wadding is.
None of this is rocket science of course but it's a fair amount of trouble.