The Central line during summer is so hot it wouldn't be legal to transport cattle on it.
The Northern line regularly has packed platforms going north in the morning.
The Metropolitan has been undergoing "modernisation" for over a decade...
EDIT trains also frequently run late OR early
They break under the slightest deviation in nature - rain, snow, heat and leaves on the track all caused breakdown in service every year
And let's not forget the dreaded "signal failure".
By comparison my experiences in Munich, China, Tokyo, Seoul, Singapore and Hong Kong have all illustrated what an efficient metro system can be. Though they obviously have 50+ years less infrastructure baggage to lit around