you can't really point to just one thing. they are a pretty useful tractor if you can find a good one and keep it running. i have one i use as a utility with a loader for misc stuff (lifting, logging, splitting firewood, back blading). I do not use it for anything that requires a load be put on on it.
problems i have had with mine:
transmission high range synchros
continual carb adjustments for the weather
eats points on an irregular basis
hyd screen filter plugs up now an then