Yes the complexity and scale of urban planning pretty much requires splitting the mesh. It is rare in single residence (except for large estates). Another case is in subdivisions where the meshes divide on property lines.
Another thing that might help in troubleshooting a neurotic mesh: Meshes seem to be primarily concerned with the integrity of their boundaries and any holes you might have. Surface contours can accept a high degree of screwiness. Maybe this helps to narrow down the search?
David Collins
Win10 64bit Intel i7 6700 3.40 Ghz, 32 Gb RAM, GeForce RTX 3070 AC 27.0 (4001 INT FULL)
Thanks everyone for your insights! Very useful and informative.
We try to keep an "archive copy" of the original mesh as a back-up on a dedicated layer - and the idea of splitting the mesh into managable, rational blocks is a good idea.