It's not even close to being ready. Even rendering a simple page like hackernews is broken.
That's not to say I am not excited for this. I think servo will be huge and I think it will see widespread adoption. It's just that with all the news we hear about it, you might be lead to think that today it's competing with webkit. It's not. Soon, though.
A: We did a ton of work
B: We're not even remotely close to something really useful yet
I think that says more about how websites work, to be honest.
(In any case, I think the complexity is fundamental to many of the specs involved.)
Edge should, in principle, support everything, including legacy sites (at least as well as other browsers do today). Trident is only being kept around for sites that do extensive UA sniffing and rely on obscure behaviour — and pretty much only for the intranet.
I guess you could run two browser engines -- one for the UI, one for the content, but I'm not sure how well that would work.
The sheer amount of work involved in getting Servo to a usable state should be less surprising when you consider that all of the browser engines in current use date back to the 90s in some form (1997 for Gecko and Trident, 1998 for KHTML/WebKit/Blink).
[0] Notice the inconsistent sizes of comments (which actually does not always even correlate with nested depth): https://www.dropbox.com/s/b47lhe3px83wq9j/2015-05-01%2019.29...
Unfortunately the algorithm they use breaks horribly on comment threads, because it decides that long comments are content, while short comments are not important. (At least, this is my deduction from seeing it in action.)
You'll see it on reddit, forum threads, and here on HN -- so this is not exactly a layout bug, but a problem with how Firefox/Chrome try to apply some simple semantics to web content.
HN displays ok in Opera Android but mostly because Opera can reflow text on zoom. That fixes almost everything.