There’s not much worth noting from this blog post (I’m disappointed, it’s a list of Starling’s app’s security features but doesn’t explain the trade off between different approaches or increased security & increased friction…maybe my expectations are too high 
) but it does mention that if you’ve rooted your phone, the app won’t work -
When you root your phone, there’s always the risk that you may have downloaded a compromised app that later accesses your valuable data, or will give your phone a virus. It’s important to us to be able to determine the integrity of your smartphone. When our app is installed it performs a set of checks to determine if the phone is safe for running our mobile app — if it’s been tampered with or modified, our app simply won’t run, protecting our customer accounts from attacks from the insertion of malicious code.
https://www.starlingbank.com/wondering-about-starling-security-heres-a-rundown/
whereas Monzo’s approach is -