When it comes to web apps, there are a lot of restrictions imposed by the browser that significantly limit the app's capabilities. That's a good thing, because it stops malicious websites from, say, deleting system32. On the other hand, it means that all web apps that need to handle files must use a not-so-elegant solution, which might not be as intuitive for the user.
For example, lets take a hypothetical UTAU replacement that runs locally, without a rendering server. The most obvious problem would be the resampler and wavtool. It is theoretically possible to get a resampler that runs as fast as the Windows ones by using WebAssembly, although that is still experimental.
But even if there was a fully working resampler in JavaScript or WebAssembly, it would still have to access the voicebank and temp files. How would that be possible if the web app can't access the file system? One alternative would be to ask the user to manually select the voicebank folder, which allows the browser to read (not write) to it. From here on, all file operations would be done in memory, which would require modifications to the resampler/wavtool code.
While Chrome does have an API that allows access to a small portion of the file system, most browsers do not implement that feature, which brings us to the next issue: cross-browser compatibility. People usually see web apps as wonderful cross-platform alternative to programs that can only run on a specific operating system. However, not all browsers implement the entire HTML5 spec, and some extend it with browser-specific features. So in order to make your web app run everywhere, you must either dumb it down or use polyfills, which add to the size of the app.
Since all file operations must be done in RAM, less powerful devices might not be able to keep up. That's why I went for cloud based rendering in my project. There are no restrictions as to what files the server can access, so by uploading your voicebank, the server can just use regular Windows resamplers to render the output. It can also run these tasks in parallel, for multiple users. In this scenario, the main performance bottleneck would be the download speed of the finished file(s), which can get quite heavy.
Long story short, it is possible to have an UTAU-like program on a Chromebook that runs locally, but it would require a powerful one and wouldn't be as intuitive. If the rendering is done on the cloud, then it could theoretically work perfectly on any device, being limited only by the connection speed.