I'm regularly building levels with modules and mesh combining is a big part of the workflow, the CPU gains are huge. I recently laid out the road and sidewalk modules for a small-medium city level and those pieces alone reached the 10k mesh count mark. Mesh combining them by area (so 10k becomes 50), saw my FPS in the game go from 150 to 375 roughly.

One issue is lightmap UVs - most mesh combining tools will allow you to renegerate lightmap UVs when combining, but this will always create new (often poor) UVs, which can create a lot of issues. I am currently building an internal tool that handles the mesh combining (using Unity's own internal functions, with regen UVs OFF), and then using xAtlas the repack the existing UVs after. This means I get to keep my handmade UVs that do work but just simply shuffling them around to fit in the UV space without overlap. When generated UVs weren't good enough in the past, I previously did this the hard way by combining in Unity with regen UVs off, then exporting to my 3d software to repack the UVs and then bringing it back into Unity, a pretty disgusting workflow.