I’m sure someone else will have a more in depth answer… but here’s my 2 cents.
World Machine is pretty much the standard landscape generation method. You can generate heightmaps or a static mesh with it. With the heightmaps you can also generate either height or angle layers (the proper term here escapes me) that can greatly speed up the initial material painting. I don’t know about the static mesh option, but with height maps, WM does also have the ability to save it as tiles. You can do it all in-editor, but I find that at least starting with a WM generated landscape gives a much more realistic base map to start with. Even with the erosion brushes I find that my landscapes done 100% in editor end up looking too smooth overall.
I would consider a landscape from WM to be “imported custom landscape” so I’m not sure what exactly you’re asking with that part.
They often need a fair amount of tweaking to get rid of the terracing caused by the lower resolution, but you can also pull in satellite heightmaps of real world locations if that’s your thing.