I agree, they had very short lifespans, despite having much time and money spent making them serviceable. The re-engining programme was a relative success, but by then, the more reliable class 20 was chosen as the standard by BR over the 17 and they werre scrapped - re-engined examples included. IMO it's just as well, they are positively HIDEOUS!
I'd nominate the class 89 as a pretty unsuccessful loco design. Yes, only a prototype was ever built, and it wasn't a great design compared to the superior class 91, and it was never renowned for its reliability - despite having a huge sum of money lavished on it by GNER to get it working again. Then it failed badly in 2001, and it was sold to preservation a few years later.
Off-topic a little, it's a pity in some respects that the 89 was unsuccessful, as a Co-Co layout AC loco would have proven to be great freight locos. Brush Traction went on to develop the Le Shuttle locos (with three Co-Co bogies) a
nd the class 92 with Bo-Bo. Was there a specification / design simplicity requirement that insisted in reliable Bo-Bo traction systems?
Of course, since the 92 no electric locos have been introduced to the UK, so on the flip side there'd be no need for extra AC locos, particularly when there are knackered 86s and redundant 90s lying in sidings rotting...
'Mon the ACs!
EDIT: 92s are Co-Co according to some pics I've looked at.. ignore the red bit!