carbon can build up needing cleaning, especially if ran at low heat for prolonged periods or you use poor quality fuel, or used at altitude without changing altitude setting (too much fuel not enough oxygen)
shuts off when overheats
can flood itself when re-starting
diesel can drip out of the intake
wiring is often too thin for 15A, so you should consider upgrading the wiring
7) infamous green fuel lines may need replacing and reinforcing
8) some models have poorly sized exhaust ducts which easily slip off even with clamping
9) supplied fuel filter may leak - consider upgrading it
10) you may need to replace the fuel pump and re-design how it is mounted to reduce noise
11) control panels are not waterproof, yet you don't want the unit inside your living space due to potential exhaust leaks and potential for fires at the exhaust outlet
can catch on fire and shoot flames out the exhaust if fuel-air mix is set too lean
if you run it by setting a temperature, the controller will run at max output until temp reached then shut down creating swings in output, the alternative is to set the pump frequency which will give a more steady output level
in a mounted unit, if the exhaust duct has a low point, water can pool and corrode it, and also the muffler has a weep hole - this should be facing down to allow water out
the remote should have a slide cover so you don't accidentally press buttons when it is in your pocket