There are widespread reports of issues in all sorts of applications. I see them regularly with our few 4K displays here at work. That means it isn't fine. If they can't deal with legacy applications in an acceptable manner, that's the definition of "not fine".
Are they running Windows 8.1?
Windows 8.1 (or was it 8?) essentially ditched the "legacy" way of DPI scaling unless applications are specifically flagged to say that they support it.
This means that non-DPI-aware applications are rendered at 100% size, and then scaled up as though they were an image using bilinear scaling. (or similar)
This is exactly how Apple handles non-retina applications on OSX.
Edit: Nearest Neighbor scaling is used at 200% scale to keep things sharp.
And there is a "disable display scaling on high DPI settings" compatibility option to disable the new scaling and use legacy DPI scaling for applications where it actually works.
The older way of handling scaling in Windows often resulted in text being scaled up but not other UI elements, which often completely broke applications.
Most of the people I see complaining about Windows' DPI scaling being "broken" are trying to use high-DPI displays on Windows 7.
OSXs high-res support, while not without some issues (cruddy fonts on older applications, mostly), is way, way, way better.
OSX only supports rendering at 1x or 2x however.
It also has an incredibly stupid "scaled resolution" option available on "retina" displays that still renders to a 2x target and then up/downscales that as an image to your monitor's native resolution.
As you might expect, this blurs the hell out of everything, compared to Windows' non-integer scaling options.