As mentioned above, your main concern would be your lack of cover by FSCS if anything was to liquidate. That being said, it would seem to be incredibly unlikely at this point.
In terms of everyone mentioning contactless transactions still going through if you lost your card, Mondo has set the card to ALWAYS authorise online, even for contactless. This does mean that you may have noticed a slight delay in contactless being accepted by retailers, however the advantage is that we instantly get the transaction appearing in timelines. But the other advantage, is that if you still have access to your phone/app then you can freeze your card and even prevent these contactless transactions. And in this instance thats probably even more secure then the current UK major banks. (Or you can always call customer services on 0800 8021 456 if you’ve lost your phone too!)
That being said, I really dont think its too much of an issue to log into the app every few days/weeks, press top-up and then use Apple Pay to top up. takes about 60 seconds from start to finish.