If you want to constructively point out a problem, then you say, hey this thing doesn't work. Not "Spartan behaving like Malware" or "Spartan is pointless because its too buggy, and I see no reason to buy Windows 10".
A web browser is really two pieces. The Rendering engine, and the "chrome" (the app that hosts the rendering engine). The rendering engine is actually very mature, and what they seem to have spent the most time on. The "chrome" is not, and that is what is still largely in development.