All the ones I have seen have been either the bearings in the (sealed) motor, the motor itself with no visible issues, or something in the (potted) electronic control unit.
Never been lucky enough to just replace a trigger and get it working again!
Both the ecu and the motor spare parts prices pretty much write the tool off.
Obviously I don't actually scrap them, they just pile up in my shed waiting for the chinese to release a £19.99 replacement motor...
I went with the Bosch brushless range and originally my set contained an impact drill and an impact wrench which came with one 8amp battery, decided to get a cheaper "8A oem" version and when I weighed them, the genuine Bosch battery was about 30% heavier than the copy, so probably more lithium. As far as I know, the "Bosch Core" battery I have also uses a different battery type in them compared to the standard 18650 ones in the others. But at 3-4 times more for the original price I figured it was worth a shot, so far, the knock off hasn't been too bad but doesn't have the capacity of the original.
Still, I do not use them a lot to the point of draining either so don't know how long the cheapy will last, but so far so good. If nothing else, it is good as a standby battery. I mainly use corded type tools but soon I will be welding up my shed, so when I am up on the frame putting the roofing metal sheets up I will predominately use the cordless tools.
I should have bought the Milwaukee brand as they have a much larger range of tools available. But once you get a particular brand you are pretty much locked in to that brand, the batteries and chargers are not interchangeable with other brands. I wanted a more powerful impact wrench and the bigger Bosch version is much larger and heavier than the Milwaukee one (and has less power) and I do not want to buy another batter and charger to suit.
So stuck with the Bosch brand for the time being. I also bought a much bigger impact drill since, as well as a 125mm angle grinder so looks like they will be with me for a while, at least they do the job.
As far as reliability, the Bosch impact wrench had a fault out of the box where the direction selector wouldn't select the right direction properly , and the charger fan failed after three charges. At least the Bosch has a 6 year warranty including the batteries, but takes the shine out of their quality control.