Blank ones I mean, with no buildings or anything. The reason I'm doing it is, after all these expansions, there are loads of buildings to use, and simply not enough room for them in the default worlds. Many of them are also really big in size which makes it close to impossible to use them. Things like the Equestrian center, Later Night tenements, concert parks and stuff.

Also what about porting things over from other worlds? Like if I wanted to move the families of Sunset Springs over to a downloaded world, is there any issues with that? Can it cause corruptions or anything (I remember at least one Sims 3 game reading that you shouldn't do that as it can mess up your save, but I can't remember if it was Sims 3 or not).

For blank worlds, the only thing you could encounter issues with is poor design: poor routing, poorly placed lots and/or lot sizes, too many/few spawners etc. However, those worlds are rare, since most creators spend a lot of time building and playtesting, often with the help of others.

You can move families from one town to the other without issues. The only case you could encounter issues in, I think, is if the family is already corrupted. You should use Porter if you want to keep the relationships between sims. Be aware that the longer you play a family, the higher the chances of corruption are - and if you transfer it to another town, the corrupted bits will be moved as well.

As for the lack of space, for the rabbitholes you may want to consider either downloading rabbithole rugs/doors and building your own, or using the merged rabbitholes included in the various EPs (like the City Hall + Military + Police combo in Late Night). You can also build your own lots and zone them as lounges, bars or clubs, that lot type does not require having the huge shells to count as a proper lounge/bar/club.

I use custom worlds for almost all of my games and the ones I've used are safe to use though Evermore Falls is very large and better for higher end computers.I've had worse problems in EA's worlds because of their overpopulation which causes lag and their lots get a lot of stuck sims.I actually use rabbit hole rugs and doors for my games when I need rabbit holes.I found the shells were usually just too big and led to routing fails for workplaces and schools.