Just to be clear, T3D is just as capable of importing World Machine terrains as UE4 and the like
You still haven't shown a screenshot of the issue so tbh, I'm still not quite sure what the issue actually is, which makes it hard to help you solve it. the smooth tool, smooths out the terrain i.e. it sort of flattens it without removing the slopes right away. So yeah detail will be lost in the progress, but exactly what detail is it you don't want to lose? What you want to do is to remove spikes etc right? So you need to smooth the spike, such that it looks more natural, so you use the smooth and a suitable brush to flatten it.
Alternatively you could use a "lower height" brush with 1 vert radius and use that to manually adjust the detail, this is pretty tedious however and doesn't always provide better results, so the smooth tool is the "shortcut".
World Machine is one of the 2 solutions people use (outside L3DT which I personally believes uses inferior algorithms, and provides worse looking, but playable, terrains. Many uses World Machine, I personally use World Creator.