In the SL Wiki there is a page titled LIMITS. This page lists most of the limits we encounter in SL. The page updated today to indicate the change from the main viewer with a new mesh uploader.
::Since viewer release 3.8.4, processing of meshes in the Collada file that have faces assigned to more than 8 materials has changed. Instead of simply dropping the extra material faces, the uploader now creates a new object to accommodate them. The result is that the single mesh is divided into multiple objects (prims) in a linkset (or coalesced object). Thus the limitation to 8 materials is removed as far as input is concerned, but still applies to each of the resulting linked objects actually uploaded. As a consequence it is now possible to upload a mesh with more than 174,752 triangles, although it will be divided into multiple objects. (Reference)
This sounds like a good change. But Drongle McMahon made this comment in my blog about the change (I’ve done a little editing on it – reference):
Oh dear. I took my eye off the ball on this one. Of the problems I reported in BUG-9015 [-[Project-Importer] Materials with > 21844 triangles cause chaos in models with > 8 materials], the worst effect (splitting object unpredictably across a single material) seems to be corrected, at least in the test case.
The problem where a material with >21844 tris [triangles/polygons] in only the high LOD causes a cryptic material to be generated, which isn’t matched in the lower LODs, [and that] still causes upload failure despite all [your] LOD files being correct. So everyone should still be advised to just avoid materials with >21844 tris. Why they don’t simply make that an explicit error [message], I don’t know.
Personally I don’t like what they did here at all, and I don’t understand the motivation. If you allow the uploader to decide how to split your mesh into separate objects, you lose control over the sizes of the split objects and therefore of their LOD switching. There is nothing difficult in splitting up a mesh (and/or materials) yourself, retaining complete control. The only people who can possibly benefit from this change are those who are uploading other peoples’ meshes not specifically made for SL, with excessive geometry and/or textures, who don’t know how to optimize them for SL. Facilitating that can only make things worse than they already are. Can anyone suggest a more sensible reason?
So, there are some problems. I have no answer for Drongle’s question of why. But, I will be keeping my stuff below the triangle/polygon limit.