They work perfectly fine in London. The network would completely fail without them, and everyone is used to them so it goes well.
London mostly works well with them. But then a large proportion of people are commuting (no luggage/children), using Oyster cards (simple, short journeys, well-designed, fast-acting smartcard system) and there tend to be enough barriers to let everybody through.
Compare that with an outer-suburban station, trying to disgorge most of a rammed 12-car peak-time express through 2-3 barriers, mostly on paper tickets (a mish-mash of season and a variety of return tickets with a baffling set of rules), some fairly unreliable smartcards that take a few seconds to load, have one person manning the gateline (and so a queue for queries) one wide gate set on two-way (god help you if you're trying to get in for the train going the other way) and a not-insignificant local commuting/leisure flow going against this tide, with a number of people trying to take suitcases and bikes through the barrier while others are trying to get out.
And they don't always work well in London, either. Try taking a couple of kids, a bunch of suitcases and an elderly granny on a Cornwall service out of Paddington, that, through various glitches, gets announced 5 minutes before departure, while the Reading commuters sprint ahead to get a seat, and the queue for the manned gate is 10 deep (and they insist you drop everything and put your ticket through anyway).
Or try Finsbury Park when there's a last-minute platform change, everybody has to head out through the barriers (half of the tickets rejected) and in through the new barriers (all of the tickets rejected) when they haven't realised they need to open them up quickly enough.
Use them, fine. But look at all the reasonable use cases, and design them well, not just "oh, the exit's this wide, we'll fit in 4, whatever".
It's not just ticket barriers that create conflict, though. Any pinch point will. Biggleswade's a good example - the steps are pretty narrow and steep, and there's a bit of a queue builds up in the evening peak. And there's always a certain bunch who feel the need to push to the front...