Both of these contradictory views are correct.
EDIT: John covered most of these points in his post above. I took too long typing.
SketchUp, when overused in a scene, is a performance killer. From my understanding of the issue there are two major disadvantages of SketchUp created models in Trainz, its heavy use of textures and high polycount, and its lack of LOD data.
Use one or two SketchUp buildings close to the track in a scene, for example (because some of them do look good as ktain.ny mentioned), then you would probably not notice any performance hit. Build the entire town out of various SketchUp models then you will. Even the most powerful GPU can be brought to its knees.
The LOD issue is particularly telling. Because SketchUp models do not contain LOD data then the graphics software and hardware will render the full house, in all its detail, regardless of how far away it is from the viewing camera. So, all the high quality detail (nails in the woodwork, cracks in the paint texture, curtains in the windows, intricate balustrades, etc) that you want rendered when a SketchUp house is placed right next to the track is also rendered when that same house is placed 1 km or more away. Even though all that high quality detail will not be seen at that distance but the GPU will still spend the same time and resources rendering it.
In case you do not know of the term, LOD = Level of Detail. This is where you, the model builder, creates extra meshes and textures to be used at set distances away from the camera view point. So, for example, you create a slightly simpler mesh and simpler textures to be used when the model is being rendered at, say, 300 metres (the nail holes and cracks are not included). You create an even simpler mesh with fewer textures (e.g. no curtains, simpler 2D balustrades, windows without separate panels, etc) to be shown when the house is 500 metres away. And so on - the creator decides how many LODs and the distances.
The advantage is that the video card software and hardware has far less work to do the further away the object is located. The disadvantage is the extra work for the creator.
I always avoid any asset that states "SketchUp" in its description. Those that do not include this warning can usually be identified by the file size - SketchUp building, for example, are usually 2 - 7 MB in size with some exceeding 10 MB while similar building that have been well designed in Blender can be as little as 20 KB in size (and yes there are exceptions).