Talking out of the back end of my body I'll post stuff / exepriences that come to mind.
Firstly I get all my MOTs done at local county council MOT testing stations. These are staffed by technicians who have generally come from both HGV and smaller vehicle experiences and as you point out they are all qualified to required levels but their individual experiences vary. Most, if not all, are both HGV, Public Service, Taxi and Private vehicle rated/qualified.
I think that possibly the big barrier between domestic cars and big HVG are pssibly the following general observations:
1) motor vehicle technical knowledge/experience between cars, taxis, small commercial and large HGV vehicles IS NOT the issue. Any competant technician in any of the motor vehicle categories can spot and diagnose unsatisfactory components and systems.
2) generally speaking I think car/private vehicle technicians (even though many I've had dealings with are complete ***** and failures) tend to have been more trained/biased towards the private consumer market. A £60 or £100 or more service bill represents a very large investment by the private customer who is also going to be thorough and critical of the service received.
3) Medium to Large fleet operators (car or trucks) just want their rigs running and drivers deployed. Personal driver complaints about a rattle here or rattle there etc. does not concern them.
4) Three above means that many commercial vehicle servicing concerns are based upon quick fixes, rapid turn-around, etc. This tends to form a working barrier and relationship between the parties.
An example.
A good battle field injury surgeon/doctor on the front line is "the best". However such a medical person could prove to be not so good in say general NHS hospital or GP practice. AND EVEN IF any such person was brilliant technically in any of these roles there will always be some person/bureaucrat who knows better. What does an ape to maintains/services a DAF HGV know about a Rolls Royce or Ferarri finess and what the owner expects!
The truth is the APES in this business are the over paid, clueless car dealer managers/boards peddling their owns careers and bonuses etc. and in reality just have no clue at all about anything let alone the technical merits and skills a good HGV/commercial technician could and do bring.
What you say about production commercial truck technology being in many cases ahead of producation car technology is also very true.
