It kind of depends on how big and complicated the worlds are. Some beautifully designed custom ones (most notoriously Alpine County, Setra, I think Vice City is one, there are others) will be unplayable for most all of us or at least become so after a while for this reason. Other simply large, rather than ginormous, or even medium sized worlds will max out if the resident population is let to grow too high. How high depends on the world and the player's system, but 300+ resident sims would be too heavy for most of us to carry and successfully stay under the RAM limit as well.
Yes, NRaas mods can help make the game run more efficiently and smooth out growth issues and of course are highly recommended. But they can't allow the game to use more RAM than it possibly can as 32-bit should it still keep calling for that much.