The limits are not in unreal so much as they are on the developer. But, You might have to rip out parts of unreal from time to time, and throw them away in favor of developing something that is scalable. Scalabel is NOT something that unreal is. And architecturally, it is not something it will ever be. If you want to make the next great MMO then you will have a lot of work ahead of you replacing unreal stuff, or perhaps wait untill Unreal 5 comes out. Which should actually be getting discussed at epic if they want to stay on top of the industry.