I have to resolve quite a few unfinished journeys every month, definitely more than three.
Most happen because they leave the barriers open at rush hour where I work and the current of people tends to push you along quite quickly so I tend to tap the card and keep walking; half the time it fails to register. The other one happens interchanging from long distance national rail to intra London national rail at CLJ where there’s no on platform reader.
I haven’t had any trouble. Always have to ring up though, it doesn’t work online. Whether that really saves me time over walking to find a ticket barrier, who knows. YMMV.