M-Tickets are also catnip for poorly trained gateline staff, as they are frequently used for advances and the standard display lists a terminal station / suggested routing. So if you purchased a standard ticket, you have to switch to the previous page to demonstrate "London Terminals" and hope to be believed. Also, a lot of gatelines are not fully equipped to accept them, instead having just a small number of gates set up for this (Marylebone is an example), which is the source of confusion/delays (I've seen people trying to tap their phone on the oyster pads!).
By the way, I notice TTL (for one) has become increasingly aggressive in trying to encourage uptake of M-Tickets. Now, M-Tickets are the default but you can "add an option" (how generous!) to collect a paper ticket. Then you have to click (for a second time) to cancel the M-Ticket and switch to a paper ticket, which if you forget to do before collecting your ticket means you are out of luck unless you can wait (potentially) 5-10 minutes. Does anyone know what the motivation is in pressuring this so heavily? Does it cost them a few pennies to issue paper tickets for pickup or something?