That they service only a minuscule part of London and refuse to take rides that take them outside of it.
Even in the center good luck hailing one at 2am on a Saturday night.
I’ve been refused rides form the Shard, Bank, and Canary Wharf plto Lancaster gate, both of which are in Zone 1.
If I want to hail a cab from my home I have to walk 10 min to Paddington and I live essentially across from Hyde Park which makes me considerably more central than most people in London.
For my friends who live in Hammersmith, Fulham, Acton and Chiswick they might as well be living in Belgium as far as black cabs are concerned they don’t go there and they won’t take you there.