I think maybe I will put something into perspective here as maybe a helpful reply and maybe not.
Keep in mind this was a goal many years ago like 35 years ago before computers had the computational power they have today and before internet existed.
I wanted a virtual reality at the time and a place for thousands of people to come and enjoy making cities from artwork and donating artwork themselves and dreamed of how to accomplish the task. I set out on my 8088 to program an algorithmic program to generate pixel data that would become a terrain.
I soon found that the memory was exhausted with something simple as 64x64 bitmap so I said someday maybe.
A couple years later I got a new computer capable of doing ‘at that time, LARGE terrains’ and internet had become a place to get information. I set out on the web and found a newsgroup that chatted about computer graphics and found out my large terrain would have to be many blocks. In the process of trying to connect these blocks I learned of a new and great OS being developed called Linux so I got involved in the Linux project and kind of forgot about my fantasy virtual world.
My job changed and I was traveling all over the world so I couldn’t work on my projects much since I didn’t have a laptop.
Anyway, in 1999 I got wind of VTP (virtual terrain project) and got somewhat involved with it which really helped me understand the process of putting together the earth DM maps and how things need to be to accomplish the paging system. In the mean time several others were already more advanced so in 2004 I got involved in L3DT.
Now L3DT as a free program is great but the pro version allowed me to go way beyond the limits and make massive terrains. However I am not a very good artist and I didn’t have the time to learn terrain design so random generated terrains became a thing of interest. I also at the time realized that to build a massive size world that I would need massive computational power or lots of time and I also needed to be able to generate model data in maps to cover the foliage and such . I also realized that to make the world I needed to be able to wrap it onto itself so it was a real world with a global look and feel. To this design I started searching for a engine capable or easily modified to make a continuous looping world with 6 degrees of freedom.
After much programming effort and a lot of sleepless nights I made a rough program to do such BUT it was based on SDL and not useable as full blown terrain system. I tried to incorporated it into Crystal Space engine and had some success but the engine just didn’t have very good layering which meant the terrain was in but the layers of splatting was limited to 4. OK, so we trashed this concept and worked on another. With Multiverse I successfully imported a 16 giga pixel in 2009 and had it up and running since it already accepted L3DT mosaics directly. The test was to run across the world but it crashed everytime I got 5 or 6 blocks into the map which meant it had a memory leak. Not having the source at the time I ditched it.
Along came Esenthal (Late 2009) which perked my interest since the engine was capable of master server and unlimited terrain. after some effort I make a tutorial on how to import insane world sizes but the engines splatting was limited too 8 layers per chunk so I found t lacking in that respect.
I started wondering about the concept since many engines I had tried needed massive work to get them through the requirements I set forth in 1981. I wasted 30 years on it and was beginning to think it would never be possible.
In 2010 I heard of Bigworlds. They were not making a development for indies at the time but the interest I had was perked enough to follow it and ask them about making a developed indie version. In 2012 hero engine was available and I took a contract for making my game. I soon learned it was rather limited in OW design but could be possible with work. Late 2012 BWT released a indie version and I was ecstatic because here we now have a clustering master server with terrains at 100x100 KM in ones zone and space mapping capable of extending it indefinitely. Well **** on them for taking out occlusion and making morph targets not possible.
So here we are with UE. Now UE can do anything you want except clustering master server out of the box but for a single player game the world can be any size! It can be 1,000,000 x 1,000,000 miles square built with 1024 16 giga pixel terrains if you want it to be but the limit is storage since as I said previously. Now the other thing is time it took me 3 months to generate one terrain at 16 giga pixel. it took 4 days to generate the alpha maps which have 43 layers of terrain data each and layer adds another 16 giga-pixels which takes space. Then you have to remap these layers to 1017 blocks and export them to UE which took over a month of work. Once you are setup with all layers the engine will run for several days importing it and if you make ONE mistake it has to go through the process again.
In L3DT you can build you foliage data maps and use them through script in UE to make automatic foliage so each map again is 16 giga-pixel and has to run through the script one block at a time which is 256 block square.
Now why I quit this was my wife of 37 years had a stoke and has been in a skilled care facility since November 27th,2015 (the day after thanksgiving) I have lost interest in game development as a whole and lost the desire to continue since it took away much of a lifetime so if you need to know the answer to ‘what can UE do?’ there it is.