airgap-raspberry-apk-signer VS airgap-coin-lib

As a side note: This is not important to most people, but we actually do the signing of our APKs air-gapped as well so attackers can't get a hold of our key and potentially sign their own, malicious, APKs with our key. https://github.com/airgap-it/airgap-raspberry-apk-signer

The audit reports have been released over 2 years ago (shortly after the audit report was done) and are available on github: https://github.com/airgap-it/airgap-coin-lib/tree/master/docs/audits

That being said, I can tell you a little bit about our experience. We developed our wallet to be blockchain agnostic from the start. All the coin specific logic is located in a shared library that is used in both the Wallet and Vault application (https://github.com/airgap-it/airgap-coin-lib/tree/master/packages/core/src/protocols). We have an interface for methods to prepare, sign and broadcast transactions, as well as generate addresses and fetching balances. To add a new coin, all you have to do is add a new class to the shared lib, implement all the logic and then add the new chain to the "supported protocols" in the apps. Everything else is handled automatically (besides coin specific features like staking, etc.)

AirGap was audited a bit more than a year ago by "Compass Security". The review is available on github: https://github.com/airgap-it/airgap-coin-lib/blob/master/docs/audits/report_85096_mobile_app_security_v1.0.pdf