The main complaint people seem to have about this approach is the large size of the executable, I don't know if Servo can make a different in this respect.
My main issue is that it's lacking some of the DOM API. It's complete enough that you wouldn't really notice when writing code yourself, but good luck finding a charting library that runs without modifications.
I initially thought this was what Azul was, but it's only just using Servo's WebRender compositor, and rolls its own CSS parser, DOM, and layout engine, so it doesn't benefit from most of the work done on Servo, and supports less CSS features.
A great example is Sciter and Sciters Note app
The x64 is 5mb impressive compared to an electron application.