One of the things I tend to hear when people work with some of the game engines out that is that they're "awful." Creation Kit for Skyrim for instance, can be kind of a clunky tool to develop with at times. Trying to cobble together a new space out of pieces of existing places can be frustrating. The main reason for this I believe is that the pieces sometimes feel like they're unnecessarily linked. Instead of having "floor" "wall" and "ceiling" pieces, they're all one static mesh which means they can only be rotated certain ways or it looks weird. Then if you do have a gap then trying to find filler that roughly matches the same style can be frustrating. In addition, with pieces being as specialized as you are you can actually start building an area and then realize that the exact piece you need doesn't exist and now you have to figure out how to fix it. In these ways I can understand how these complaints are valid to a certain extent. Of course, you can always make your own pieces but that also requires quite a bit of prior experience and skills.