In my experience the gas circuit is a pretty reliable part, though in general your compressor will almost never fail, unless you have had a period of sustained overvoltage, as the typical failure mode is built in, as the manufacturers use a steel welded wall pipe to make the refrigeration system, using the steel for all the evaporator and condenser circuit, except for those small parts that might be in the freezer with a coated aluminium coil. Invariably the steel pipe will rust, especially in the condenser and evaporator, hidden under the urethane foam, which in the condenser breaks down with time to form an acid sludge which rusts through the coil. You will see, once you destructively strip the case, the rusted section by the hottest part of the condenser, and a similar part by the evaporator start, where water condensed to corrode the steel.
This is impossible to repair, as the coils are foamed in place during assembly, and the only copper in the system is perhaps 20cm of 6.3mm tubing joining the compressor to the steel, and 30cm of capillary tube by the drier to provide the expansion device. Once they start to leak, you are going to buy a new one, though I can still get the condenser coils for old chest freezers, and the evaporator on them is copper pipe, which lasts a long time.
New fridge or freezer, the coils are integrated and the whole appliance is disposable, though I tend to keep the compressor around after pinching off the entries, as just in case spares for testing. Yes the PTC starter can fail, though they are common, and you can tell it is dead if it rattles, as the PTC element inside has shattered.
Inverter fridge or freezer there is a 10 year warranty on the compressor alone, because the manufacturers absolutely know the faulty ones will fail in the first week, and then the unit will be replaced complete. After the 1 year statutory warranty the most common failure is the electronics, which are as expensive as a new fridge, so they are rarely replaced. Most manufacturers will only sell you a new inverter board with a compressor, and you replace both, as they are unable to guarantee the board otherwise, as a failing board will send off the compressor, and likewise for the compressor. Your spares then com in at 70% of the cost of a new unit, and there is no warranty the rest of the unit will last another 5 years then, as the steel pipe will be well corroded.
Yes flammable refrigerant, but in most cases the same volume of refrigerant as in 3-8 cigarette lighters, around 60- 150g, and you can buy lighters in packs of 50 all over at the China mall, where the pack is cheaper than buying 5 retail. Plus any leak is in general slow, 1g a day at start, so you find the unit starts cooling poorly a week before it stops entirely.