Would this be any different than playing on a server?
If you're willing to go through the effort of assembling 50 people, and, more importantly, if you're willing to split the loot 50 ways, then I say that's fine. The bigger problem is bringing in an higher tiered character from another world. That is to say, the problem is essentially bringing in an auto-kill button. Even if you're using a Blizzard Staff on the EoC, it takes effort, however little.
If anything this would be really helpful to people who have trouble with moon events.
This is true, as you would have to have X sets of equipment, making it take more effort. Problem is, with the way it's currently suggested, character spawning is available from the very beginning.
You can also cheese bosses with gravity potions. Are those OP?
You can't cheese bosses, because cheesing implies that exactly zero effort was spent. Sure, you can make it a lot easier, but you still have to have a degree of skill, and put some effort in. An example of cheesing would be a lava pit during the Pirate Invasion.
In order to make their ability to fight balanced, the world-crossing would have to be introduced post-Plantera, when all major gate bosses have been defeated. I don't think the Duke would be a problem, as fighting him takes a degree of skill beyond simple NPC AI. I don't think Golem is a problem, because Golem is kind of a joke.
Now, I don't think that we should limit PNPCs until after Plantera. We can, after all just treat them as aesthetics.
I will say, a more efficient way would be to have the PNPCs spawn if they set their spawn point in the world using a bed.This would also choose their room for them.