Nobody who cheers "mobile tickets" has ever watched a guard or ticket inspector do their job. In the UK, a guard with good eyesight and a sprightly, cheerful manner can almost run through the carriages shouting "tickets please!" and see, with a single glance, all s/he needs. Then she gets to the muppets with phones. They faff, fumble and fuss. They scowl at being interrupted watching videos, browsing social media and…
I feel the same way about people who use mobile phone NFC payment at ticket barriers on the tube instead of a plastic cards (again via NFC) Using your phone is much slower and there's a significant chance it'll just fail and hold up the person behind you (me).
There used to be a time where the oyster card was slightly faster than the phone in terms of registering at the barrier but that's improved a lot over time.
I don't live in London anymore but when I've visited it's been so good just tapping your phone through everything.
Additionally the hold ups at the barriers in my experience are more down to people's misunderstanding of the cadence required between each passenger. This applies to both oyster/card and phone, and is more of a UX problem of the barrier rather than the technology used to activate it