Like so many other things, there is tremendous variability in coil longevity. I've had a few fail in service on my TR3; what was probably the original 1956 Lucas coil on TS13571L broke down on the way to Triumphest 2009. Didn't fail completely, just made the engine run bad (lots of misfires, lack of power and so on). I had just gotten into town when it started acting up, and finished driving to the event that way.
Only coil I found at the event was a Lucas "Sports" coil. It only lasted about 2 years before it failed, on the freeway during rush hour. A much "harder" failure, I did manage to coax the engine to run again, but it took full throttle just to keep it idling and I had to walk away. Initially, I thought it was the condenser, but replacing it made no difference at all.
I've had a couple of failures with GM "electronic" coils, too. When I bought my Buick wagon, it was scared of water and would refuse to run if it even caught sight of a puddle! A new coil solved the problem. Years later, it flunked a smog check and the problem was (eventually) traced to the coil.
Ken Gillanders (who ran British Frame and Engine for many years) once told me he bought a whole case of Lucas Sports coils and tested all of them. He claimed that over half the case was either DOA or tested weak. And he refused to sell the others, discarded the entire case. Ken was a class act.