The biggest drawback is that it only supports a subset of Web APIs, even the DOM APIs. Most sufficiently large libraries that do more than pure compute will use an API that isn't supported, so you can't just import a random chart library. Of course fixing that is a sisyphean task for a single-developer project like Sciter.
I still really like it, but if you use it be prepared to either modify the libraries you use or write your own code.