How is scalability of walletnotify of the full node wallet
Using walletnotify has proven to work and is used by a lot of scalable apps today. I would not rely on it completely though. walletnotify's job is to execute a shell command every time it receives a wallet transaction. This means data is flowing between processes, which does not make me entirely comfortable as a developer. After all, it only takes one error for someone to be missing their funds.
I would recommend not only using walletnotify, but to also check X blocks after Y minutes. You can keep track of blocks passed and if any transactions were not processed by walletnotify, your cronjob of checking transactions will recover the transaction. You can use listsinceblock RPC command and keep track of the block height each time you check transactions. You could store transactions processed in a DB somewhere.
After all, this is a financial application so I think a backup check is vital.