Is this platform adequate to build a web app?

You may want to focus in on whether SS has the flexibility you need on the backend. Since SS gives no access to the backend that could be a big issue.

On the areas you mention SS is a consumer level product. Not a developer level product. So for example payment options are very limited. PayPal and stripe are fine for what they do but there is no option to include your own, no access to the backend.

If you are thinking of developing some kind of web app SS might be able to host some of the elements of it but you may need services like AWS (Amazon Web Services) for parts of the app.

I think the issues you might have are not limited to SS. Any service that is targeted to getting average folks making their own sites is by definition limited.

If you do decide to go with SS. Do take a look at v7.0 Developer Mode. Again this doesn't open up the backend to you but it does have some deeper options, at the cost of not being on the latest and greatest of SS's offerings. The Brine template family is a pretty good choice for many projects on v7.0.