First thing that wears down quick is the tubing inside the tank(s)
Muffler bolts cracking is something I never heard before. I'm using a gasket though so it maybe different if you don't use a gasket.
Many parts have only limited life. e.g. glow plug, wooden blades, bearings, croc connector on glowplug.
Most bearings will last at least a few hundred flights, assuming you greased them before assembly. Without grease, esp the high speed bearings in the engine area will break down quick. The engine rear bearing is also wearing out quick, depending on the engine and other factors. Sometimes it lasts only 60 flights.
If your machine has a vibration problem, all parts will suffer.