I am planning a webapp to control bank paying orders. In a quick review, the user goes online and creates a payment order. This order goes to other people that pays it and register the payment on the system. The system keeps track of all the payments, keeping the account balance up-to-date. The system needs a login system, bank integration, and to support at some point thousands of clients.

We can find articles on the web about the benefits of using wordpress platform to build webapps. However, I could not find discussion with counterarguments to user wordpress. As the platform the most important choice in webapp project, I would to know more about the pitfalls and harms for choosing wordpress.

The question is: What are the benefits and harms for choosing wordpress as a development platform for a webapp that need to be integrated with other system (backend systems) and to handle thousands of users (does it scale up?)?