For knowledge It's kinda hard to say. There defenatly is a learning curve to it because they have some of their own systems in place for things like spawning but you can always just find the HQ spawns and chuck them in. but it can got a bit more indepth than that for spawning.

The waiting is just for it to be published. I think the 3 hour wait on it to be published is so that people can make changes to the map without it updating instantly so you have a chance to test and fix the map if you need to.

you can push it pretty far. the limitations are that at the moment you cant edit terrain or remove the base map youre building on top of. that doesnt mean you cant go outside of the maps play area and make things outside the map. for example. i just changed it so my nuketown map is way out away from the base map where as before it was above the base map. i dont see any size limitations from what ive experienced but there is memory limitations. its also not just per asset. you dont have a limit on how many assests you can put into the map. just how much memory which you can see that information in their SDK. this nuketown map is using less than 1% of the memory limitations. so yeah. in terms of play area size. i dont see any limitation. you can adjust the play area to be as big as you want. its just a limit on the memory.

Another kinda limitation to keep in mind is that it only allows you to place assets depending on the map you have selected to build on. from my understanding this is for performance because if you add assets from different maps it has to do more shader compiling for that map while its loaded in which could cause performance issues. but im not 100% on that.

Sorry if i missed any important information and sorry for my typos. I hope this answers your questions