If this long list of problemsfrom an ex RN submariner is true the British submarines seem to be a disaster waiting to happen. It’s doubtful his sub could even have launched it’s missiles if it had to:
One shudders to think what the Russian boats are like…
It is the only instance in the history of naval warfare where one submarine intentionally sank another while both were submerged. Nothing in the Pacific campaign and nothing since.
To the OP’s question, the weakest point, as with most complex machinery, is the operator. human error.