I just downloaded Brave on my new iPhone X and noticed that if I activate the Browser lock feature (which makes Brave ask for a PIN at every new session) the app crash and then is gonna be impossible to open it again without an immediate crash following. So basically you are forced to cancel the app and re-download it. On My previous iPhone 7 I was able to activate this feature without any problem.
I logged an issue for this in the mean time: https://github.com/brave/browser-ios/issues/1610
We fixed it. The change will go live in the next app release.
Thanks for your report.