Any Chromebook more than $400 right now is just there to absorb even more disposable income from the market, because they ship with nicer finishes and faster processors, but they are anemic on memory and storage, making them subpar for intense multitasking, or getting certain types of work done. The situations in which they excel can be reliably hit by Chromebooks around a $400 price point, and even cheaper Chromebooks allow one to forego performance for the increased disposability of the machine.
I suspect one reason for $600 devices, other than Google itself trying to reposition Chromebooks as more upmarket, is because $600 Windows laptops are still are a mess of preloaded bullshit put there by the vendor. Microsoft has tried various ways to fix this but it only tends to protect high-end models. And build quality and the nature of hardware compromises at that price point have always been unpleasant, save for a few concentrated efforts like Lenovo's IdeaPad line. In other words, $600 laptops are second only to $200 laptops in making Windows look bad, making them an easy target for an OS that proved that $200 laptops can actually be quite good.
Still, there are few challenges. Chromebooks' filesystem paradigm deemphasises local storage to the point of cumbersome, relying on Google Drive or custom interfaces and implementations built for each app that know how to pull up past work. This is a smart idea when everything works, but makes import, export, and context switching harder, and makes sharing a feature of the product rather than a file-based affair.
On Chromebooks, Chrome's fantastic profile system is deliberately conflated with Chrome OS login sessions, which makes it harder for one user to maintain multiple independent browsing contexts than when using Chrome on other platforms. Power users on Windows can run multiple browsers, or use profile systems in browsers to keep separation, but on Chrome OS, you only get two de facto contexts (the white one and the black one with the cool spy icon), you blast the same cookies everywhere, and half your builtin applications are just hyperlinks to auto-log you into the corresponding Google product in your global white context. Applications like Hangouts (the app, not the extension) are rare, where the entire window inherits your OS login, but keeps your context entirely separate from what you're doing in the browser.
Nonetheless, with Service Workers and graphics APIs and auto-resuming applications and unintrusive updates, and people using Google products anyway, Microsoft should be worried, because they're being challenged for customers in a segment where their OS is least compelling, and was largely used by default.