I'm getting an unexpected behavior withe the Smooth SOP - I've turned it way up in order to illustrate the point and compared it to what I get out of the Sculpt SOP set to smooth, which is much closer to what I'd expect (see below, file attached). I'm getting a weird bias towards the corners in the cube example which creates an unwanted bulge in the surface, and the issue presents itself in every circumstance to varying degrees. Tweaking the values changes the result but never completely eliminates it. The problem seems to manifest itself as geometric curvatures approach 90 degrees and beyond.

set cutoff frequency to much higher number, like 3 or something

That's better, thanks! I didn't think that anything over 1 would do anything useful, given that a value of 1 produces such a strange result and the default is .1.

I usually use either 0.1 or 10. There is some value where the geo tend to explode, around 1-2 something I think.

I wonder why it goes nuts at 1-2, but works at values lower than 1, and higher than 2?