I have worked with both as well. Mostly Espar. I think a good part of the reliability is in the installation. If you have your intake where it's not sucking dirt, the exhaust where it's not going back in to the intake, get the fuel from a filtered source and follow the directions they should last and be fairly reliable. Where they fail is plugged nozzels, gelled fuel in that skinny fuel line and sooted up burning chambers. Oh yeah and don't mount the damn thing as a mud flap, if you can keep it away from the elements then all the better. I have seen many mounted on the frame rails of larger trucks getting blasted with sand, ice snow.... those onse seem to get the modules replaced every year or so... go figure 
