Responding to myself since multiple asked:
Slow: many many journeys via bus are usually at least 4 to 5x slower than driving. Typical example for me to go to the nearest large supermarket from where I live (approx 2 miles) is a 7 minute drive (so average 18mph) and pennies in fuel/energy costs or a 44 minute bus journey with a 9 minute walk at the end (each way) that requires changing buses half-way and costs £1.75 x 2 = £3.50 return unless you get a hopper fair. it's actually quicker to walk-tube-walk (38 minutes) but that is I think £4.20 return just for a quick shop at the big Tescos! It would only take 41 minutes to walk the whole way, so a bus is actually slower than walking.
Unreliable: just this week the tubes all stopped due to a "power blip" meaning I missed an appointment. This morning the train I was on changed it's destination after I got on it, meaning I had to get off and go back to a junction and get another train, which incidentally I had to wait 7 minutes for and it was packed. There is a joke about "TfL minutes" because the minutes they show on the countdown clocks at bus stations and tube stations are typically significantly longer than real minutes. There barely goes by a month without at least the threat of industrial action either, meaning you have to reschedule things just incase (and usually these are called off at the last minute meaning the union got what they wanted, TfL can say they avoided the strike, but you and I have already had to alter our plans or rebook meetings or pay for extra childcare etc already but because it is called off within 12-24 hours you are too late to cancel your own plans without penalties etc etc)
Dirty: look at your hands after getting the tube and holding onto a handrail. Or blow your nose. It's dirty. No one can deny this. The air in the tube is worse than the air on the street in terms of PM2.5 etc - there have been many studies on this
General awfulness: try getting on a rushhour train. If you can physically squeeze on to a train that is. And if you do, it's a pretty unpleasant, sweaty, gross, frottagy horror story.
People who think that it is this amazing nirvana are deluding themselves. Are there worse places? Sure. But let's not delude ourselves that it is by any regard "good". I guess if you are visiting from somewhere else on vacation then there is a novelty-factor, but if you have to do the 9-5 grind for decades at a time, if you have appointments to keep, a boss that yells at you if you are late, kids to collect on time from school (or they grass you up to social services because you can't answer the phone because you are trapped underground), or anything that requires a cheap and reliable service that you can count on then it sucks.