Which? Have just published their overview of the challenger banks.
Aside from saying that Monzo has 200k users, it gives a pretty good summary of Monzo
Edit - I spoke too soon
Also, Revolut is the only bank on this list to offer GPS security, so if someone uses your card to make a payment you’re told where the card is being spent so you can see any discrepancies.
(Monzo & Starling do this or at least, the equivalent too).