Recently we developed and published a mobile banking app on the app store, for a big banking organization. The bank hired a security firm to perform ethical hacking over the
There's always a risk involved. Even if you don't introduce vulnerabilities yourself, the platform may allow for exploits which in the end may offer an entry point for a malicious attacker.
As to your question: It is not safe to assume that a hardcoded URL, even if obfuscated beyond belief, can't be peeled out of your product. Always design your apps such that safety of user data is guaranteed (as far as possible) even if built in ressources get compromised. If the knowledge of that URL alone poses a security threat, then your whole approach and your clients API is inherently insecure. Remember that such information could possibly be captured by a man-in-the-middle attack (and other modes of attack) as well.
Avoid security by obscurity. Store sensitive data only on disk if it is necessary. As a rule don't allow PIN / TAN storage.
Some thoughts which may (or may not) convince your client that your app is as safe as it can be: