And Ryanair would charge you? how much? for a tablet/phone boarding 'card'
Well, the cost of the tablet, or £/€60, up to you.
As long as they operate using however many handling agents from some pretty small airports, their one-size-fits-all approach means they will want to be able to see / keep a paper copy -or at least the tear off bit.
Now if you want to tear off half your tablet and let them have it, then fine. Otherwise stick to paper. I always have a spare copy in case I end up on any muggers' alley on my final night / day before departure.