It seems the Steem team are in no rush to deliver the wallet they showed us at the last Steemfest, so this is good news. I think Sneak was saying it would be much more secure on Apple, but true security is always hard to achieve.
Now if only there were places I could go and pay with Steem :)
So this app uses AES256 encryption just like for example the original bitcoin wallet, and the Steem cli_wallet app. If you decide to store your active key (no need to do that if you don't want to use the 'send' feature). You will need to encrypt that active key with an obligatory strong passphrase. But optionally you can then store that passphrase inside the socalled 'secure enclave' a seperate physical storage area on the ios device, should the device support Touch ID or Face ID.
That's great, but the security risks tend to be around the edges. A user still has to access the encrypted store somehow and other apps could potentially compromise things. I'm no expert on this, but I've heard people like Steve Gibson and Bruce Schneier talking about how hard security can be. I'm not trying to put a downer on your project. We have to be realistic about the risks. I'm sure others will be checking your code and I hope they can spot potential problems.