Even bigger drawback is that it ain't Free, honestly. Sure, the licence allows you to use it if the users install the sciter runtime separately, but that ain't really am option beyond development imo.
Of course static linking would be nicer, but I think it's fair to charge money for that (as well as for source code access). The project has to finance itself somehow.
Incorrect. Linux doesn't care where you load a .so from. You could load it by using a path, or you could add a path to your LD_LIBRARY_PATH env var. You could even add it to your compiled binary's RPATH. All methods support relative paths. The latter requires a binary, but the first two work with nearly all scripting languages.
Also think about a software tool that costs say $100. It ain't free but that's your hourly salary Mr. Professional Developer. Using only free tools is penny-wise and pound foolish.
The trouble is that everyone wants to be a landlord these days by charging rent instead of allowing you to buy a copy of the software.
Every new feature also adds complexity and size, so it goes exactly against one of the main advantages of the project
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.