There are many aspects where issues can develop with custom worlds such as: The following additional, specific notes are available for online worlds: Replacing a World that has been Updated.
This topic contains a few details on some of the issues that players might experience with custom worlds, as well as what we have done to try to address such issues.
That is all in addition to coming up with a design and doing what is typically months of building for anything other than a tiny world.
To the casual observer it probably seems much like building a lot but on a much bigger scale, but there are a huge number of additional considerations, such as the effect on game performance caused by spawners, effects, terrain mesh density, how close together individual lots are, quantity of textures in addition to things like determining Sim and camera routing, organizing the world layers so that flora can be rendered as efficiently as possible, conforming lot edges to get rid of unsightly gaps, considerations about the transport network, the balance of population verses the number of available jobs etc etc.
