I think that it just uses the system webview, whatever that is. I don't think it offers "dynamic" switching between multiple implementations.onnthe same platform.

Did it? Perhaps it's easier to support crossplatform things if you can control which features of all the engines are being used.

Doesn't it use WebkitGTK, at least on Linux? I expect it has the same problems, but it's less of a problem for developing UIs than for browsing to arbitrary web pages—it might get a bit annoying, but you can always work around the inconsistencies or limitations by tweaking your own HTML/CSS/etc.