Thanks
Yes I agree, but the scope of our project is much smaller than Outerra's, we only aim to provide flat environments for a video game with this. There's no dynamic download of DEM data to allow the player to go around the entire earth, etc, (which seem to me very complicated to implement and maintain), and all the other stuff Outerra offers as a software. I don't know if it would be feasible to get it all working inside Unity.
About issues with Unity, we ourselves have faced some limitations which we had to work around, some others that we can't do anything about... The Unity API not being thread safe is the biggest drawback, there is some stuff I would much prefer to be doing in my worker thread instead of having to let the main thread do it (creating and filling up meshes with data, creating mesh colliders). There is also no way for us to have the terrain data existing only on VRAM and being rendered from there (which is I believe what Outerra does ?) we are forced to get the generated terrain data arrays back from the GPU to create Unity Mesh objects to be able to render the terrain, which also uses a lot of RAM. We had some trouble with Unity's old implementation of Mono which uses an old, inefficient garbage collector, and we'd much prefer to be handling the data with pointers ourselves at this point. Unity also uses by default a traditional depth buffer which mostly give us unstable shadows with their cascaded shadow map system with huge distances (but implementing a reverse-float depth buffer is on their road map so fingers crossed, or we could do it ourselves as I've seen others do)
But it is still an incredbly flexible engine, I'm amazed by what it let us do with it to this day. ShaderLab also makes for very easy shader learning and programming. But I'd say that something entirely like Outerra within Unity would be a difficult thing to achieve.
Edit: I've also uploaded a video (which hardly survived youtube compression even though it looked good before uploading...) if anyone wants to see it in movement :