My car has had several transmission/drivetrain type jobs done on it by a garage with very good reputation and in truth most of the large nuts and bolts were just done up f-tight with an air gun. Exception being the wheel nuts and maybe the flywheel (job was new clutch, flywheel and LSD fitted). Same for when I had a driveshaft boot replaced, which was driveshaft out of the hub etc. They'll have done literally hundreds of jobs the same way and not had problems.
While manufacturers specify torques for absolutely everything, in reality a lot of things just get done up tight and it's never an issue. In some cases it'll actually be hard to find the correct data for all the nuts and bolts, then there will actually be a range of torques which work for some fixings and the manufacturers setting will actually be picked to match other tooling on the same assembly process. Once you put lubricant anywhere near a fixing. that throws all the torques out too as it alters the friction component of the torque. Learned this through years of working in the car industry. And especially on chassis components the torques used in manufacture are pretty complex, combining torque and a follow-up angle which takes special tooling to achieve taking into account access etc.
Myself when I'm doing a job on the car like brakes etc I'll try as much as possible to stick to the specified torques, and wheels ALWAYS go on with a torque wrench.
On the other hand for the bike engines my brother works on, the torque wrench is kept busy on most of the internals - big ends, rod bolts, cam caps, rotors etc but not for external casings etc.
it's what random breezeblocks/bricks laying about come in handy for 
