Early CIS type injectors had the highest failure rate, and some failure was a matter of perspective. Early versions of the black injector seals would leak air, as would the inserts that secured them in the heads. Cheap replacement green seals leaked just as badly. Bosch green seals had no issues. Many fuel injectors seeped fuel when checked under system pressure. The fuel pressure would drop in the fuel lines after shut down and if the engine was hot, the fuel would boil. The result was a bit like a vapor lock. Hard to start, requiring many tries, and ran rough until each injector line refilled and was brought up to pressure. An identical symptom was generated by anything that could lower the system pressure in some versions. There were check valves on the fuel pump, in the fuel distributor, and on some fuel filter banjo bolts. The fuel accumulator could take care of slight seepage, keeping the pressure up until the motor cooled.
As to electrically operated fuel injection, we see and have seen, very few injector failures. Water in the fuel during the 80’s and early 90’s caused internal corrosion of the injectors, and everything else in some cases. Early CIS fuel injection systems lost fuel distributors, and sometimes the entire fuel system. Poor fuel quality, which is not related to low octane, causes varnish inside the fuel system and vapor systems, as well as engine carbon (except FSI) and crankcase grit. When we see modern fuel injection systems with injector failures, fuel quality is usually the cause. Occasionally, we find water in the fuel because the fuel tank vent system leaks. For most modern cars, that problem is over. Most cars made this century have a system to check the fuel tank and vapor lines for leakage each time one starts the car.
EXCEPT for injectors in FSI engines: If an electrically operated fuel injector barely weeps under fuel pump pressure, that is gets damp but the fuel pressure does not drop below specs after shutting off the pump, we generally let it go. If the injector seeps, and the fuel pressure drops off below spec slowly when the pump is shut down, then we may replace it, but it might not be urgent. If the pressure drops rapidly due to the injector leaking, then it does require replacement. If the injector leaks, i.e.; sprays under pressure without a signal to open, then that injector is history and MUST be replaced, as the motor will NOT run correctly.
We are not altogether sure about the failure rate of the TFSI and FSI injectors that go directly into the combustion chambers on those engines. Currently there is no way to see them and have system pressure on them at the same time. Nor is there equipment to test them on the bench either, yet. We have seen failures, but generally when we find mechanics have tried to change the seals, or to clean the surface of injectors, and damaged the extremely tiny holes. There is a specialized tool kit for replacing the injector seals on these injectors. Use it or weep.
We have seen FSI injector failure from leakage. With a fuel system pressure that runs between 725 to 2175 psi, the engine and the exhaust system fill with unburnt fuel in a New York Minute. A bit dangerous, as the engine is blowing vast amounts of evaporated gasoline from the tailpipe as it chugs to a halt.