I do not believe there is a hard limit on scalability, but be reasonable. Don’t expect to fit millions of users on a MySQL database.
Another bottleneck is the server resources. Every JSON API call requires some CPU, memory and disk resources. You can minimize this by slowing down or disabling frontend polling.
But if you are planning to have hundreds of thousands of users, that means that your business is large enough that you can hire a team of developers to build you a brand new system from scratch. No need to base it on WordPress. There are far more efficient platforms out there, the advantage of WordPress on a LAMP stack is its ease of use and large user base, not its performance.
I am guessing that this plugin can be used with up to a few thousand users on a powerful server. YMMV.
with regards