I gather from what you said, that engineers actually capable of diagnosing boiler faults are few and far between,
More accurately ""installers actually capable of diagnosing boiler faults are few and far between""
The RGI has a qualification that suggests he or she has the ability and knowledge to be able to install gas piping and certain types of gas appliances to a certain standard of safety.
Most can follow the manufacturer's fault finding flow diagrams which will identify the part most likely to need replacing for about 90% of the single item faults.
It gets interesting when fault is one of the 10% not covered by the fault finding flow diagrams.
It gets expensive when the fault finding flow diagrams suggest replacing an item which has been damaged by a fault on a different item. An engineer trained / experienced on that particular boiler is likely to check why the item failed before replacing it. This avoids the new item being damaged as soon as it is fitted and the boiler tested.