Actually, no, the mods do not fix everything. If they did, people wouldn't be mentioning that their games are freezing or lagging. :wink:
But just reporting a freeze or lag does not help narrow down the cause. What I try to do is to pause the game and go looking around the world to see if I can find the stuck sim or issue. I have even used Twallan's Home Opener for this although I don't have it installed atm. But it lets you look inside every building, even private residences.
Personally I think it's good if a mix of people play, those who use mods and those who don't since both can spot errors either way, and both types of players will be playing the world when it's released. But if creators prefer that we test without mods they can always say so. :-)