The Unsharp Mask filter works by evaluating the contrast between adjacent pixels, and increasing that contrast when it's relatively high. The idea is that a large contrast difference between adjacent pixels usually represents an edge. But the filter doesn't really recognize edges, just pixel differences, so successful sharpening requires finding the settings that accentuate the edges in the image in a natural-looking way. One important implication of this is that optimal settings for Unsharp Mask depend primarily on the image content, and secondarily on the resolution and the output process at which the image is aimed. Close subjects with soft detail need a very different treatment from distant subjects with lots of fine detail.
Perceived sharpness is a function of local contrast.
Another key point is that judging sharpening from an image's on-screen appearance is quite tricky. Always sharpen the image at the final output resolution. The key parameter in obtaining good results from Unsharp Mask is the Radius setting. The Radius needs to match the image content. That said, a large Radius setting produces a larger halo than a small Radius setting.