We are currently using the Monero library from the wonderful people at Cake Wallet (who walk the walk and made it FOSS btw). But we do things a bit differently than them under the hood, and the lib isn't meeting our needs. In particular we can't compile for Windows (blocking our release of our desktop app), and we are having a major bug on iOS that causes crashes. Too many open files say the logs (if you're an iOS dev and want to help out, do let us know).