Nira is using some kind of virtual texturing system, where one big texture is streamed in smaller chunks to client. For them it is probably easier to process one big texture, but should be not problem even several smaller. From web server perspective, every texture(or even smalle file) is creating one request from client to server. This is problem on web pages like forums where are many small images(e.g. icons, banners) and where a lot of people are reading pages at the same time. I don't think there is so much pressure on Nira server in comparisson some webhosting server with a lot of web portals, which are people visiting in hundreds/thousands every second.
Generaly, smaller textures are necessary for speeding up 3D rendering(games, game engines). GPUs have VRAM and caches. If texture(and its smaller versions called MIPs) can not fit in small cache(older GPUs just few MB, RTX 4xxx 24-72MB), then they are sampled only from VRAM, which is slower. For one model and simple shader it doest not matter much, but if you use normal maps, diffuse maps and other maps, then it start to slow down rendering, especially on low end desktop GPUs or mobile GPUs.
If you need model of building for inspection(cracks , etc.), I would use just point cloud. If you need high poly 3D model with textures, then you can try network local processing, where all tasks are split automatically on smaller chunks, which cost almost no RAM. Here is great video about that
https://www.youtube.com/watch?v=BYRIC-qkZJ8About high polycount model. If you have good viewer(e.g. tilled, or model is fully loaded in GPU VRAM, it is fast) and you do not need to edit that model, then you can use 200M poly model, or even with higher polycount. But if you need to edit model in DCC apps, then even 4M poly one chunk is problem(selecting polygons, moving them, using undo level system, ...)
RizomUV - I am using automatic model splitting and creating UV seams -> islands, where I can decide how big I want them based on the distortion. For 200M poly model in one chunk this would take several hours. So high poly model is good only for creating normal maps from high poly to low poly and in this case you need UV mapping only on low poly model. High poly one do not need UV mapping at all.
To learn RizomUV it is good to use their official tutorials on youtube.
This is my largest hobby project with info in first comment
https://www.reddit.com/r/photogrammetry/comments/1c8rg4j/camera_fly_over_the_old_unfinished_cable_car/