The issues are:
1) Injectors on earlier vehicles. They are programmable to sync with the main ECU. They fail and stop injecting. It is an internal electronic fault, hence the individual injectors are irreparable (at the moment, as some people are investigating and looking for a solution). Isuzu still honour the free recall on this issue to fit the latest generation injectors which seem to have resolved the problem on the whole. If the truck has had the recall work done then you are on your own and have to fix it yourself. Possible for the average spanner monkey, but you will need a Tech 2 to program the new injector to the ECU.
2) Injector sleeves (shudder). The original sleeves have two o'rings on them. These o'rings separate the oil/coolant/diesel galleries on the common rail. It is a Caterpiller design that was incorrectly toleranced. It lead to the o'rings leaking diesel into the oil gallery. The diesel is under greater pressure at this point. The first time you notice is when the oil level rises. The oil gets thinner and the first thing to go is the seals in the turbo. The diesely oil then gets dragged through the air inlet system. Second and subsequent signs are the turbo starting to give notice as it isn't get the proper lubrication. Final notice is the seals totally going in the turbo. Clouds of smoke, but more dramatically the engine running away as it self fuels itself with the oil. Take the key out and it will still run until it seizes or pops a cog. You need a special tool to remove the sleeves. With that tool it isn't that difficult a job to do. Without that tool near on impossible. People have successfully made their own tools using an old sleeve to go by.
3) ORPS (Oil Rail Pressure Sensor) This reads the oil pressure. It relies on high oil pressure to start/run. The sensors have a tendency to fail. Can be diagnosed by cutting the "white wire". This kills the signal it needs. Without that it will allow the truck to start as it runs in a "limp mode". Fiddly but doable. You do need a Tech 2 to sync it with the ECU properly.
4) HPOP Valve (High Pressure Oil Pump). This opens and closes delivering oil. They stick shut. I reckon old cruddy oil due to lack of changes of the oil and both oil filters. My theory is the dirt scores and damages the internals of the HPOP Valve. When the oil pressure drops when you switch off the valve inside shuts and sticks preventing the truck from starting again. No oil, no oil pressure, engine isn't allowed to start. Fiddly but straight forward to change. No Tech 2 required.
5) Oil pickup pipe. It snaps off its mount plate and falls into the bottom of the sump. Guess what. No pickup, no oil. No oil, no oil pressure. No oil pressure, no start. Diagnosed by putting an extra 2 litres of oil in the engine. This lifts the level above the mount level so it can be drawn up and pressurised, allowing the engine to start. Possible to change with the engine sort of in situ.
This does sound like a nightmare, but in fairness with some knowledge and a Tech 2 it is a great engine. Many many people have these engines and they run fine all day every day. It is just when they go wrong it is a pain.