I've once implemented the basic idea behind the algorithm used in slug(described in the paper [1], though without the 'band' optimization, I just wanted to see how it works), and I agree with you, the real innovation is in that quadratic root-finder. It can tell you whether you are inside or outside just by manipulating the three control points of a curve, it's very fast, what remains to be done is to use an acceleration data structure so that you don't have to check for every curve. That works very well for quadratic Bézier curves, in the paper it says that it can be easily extended to cubics, though no example is provided(and I doubt it's trivial). What I think would be hard with Slug's method is extending it to draw gradients, shadows, basically general vector graphics like you say. Eric Lengyel on his twitter showed a demo [2] using Slug to render general vector graphics, but I'm not sure of how many features it supports, but it definitely supports cubic Bézier curves. I'd also like to add that the algorithm didn't impress me with how the text looks at small sizes, which I think is very important in general, though maybe not so much for games(maybe I just didn't implement it correctly).

[1] "GPU-Centered Font Rendering Directly from Glyph Outlines" http://jcgt.org/published/0006/02/02/

[2] https://twitter.com/EricLengyel/status/1190045334791057408