The bug that stopped me was in the game engine (not the graphics engine). We don't have the source code for that. I could translate the EXE into machine code and read some of that, but not easily enough to track down the problem.
The problem had to do with the targeting routines. I forget the exact details, but IIRC sometimes the AI of a certain monster/NPC would not change target status properly and the game couldn't proceed. I suspected it was related to pathing and line-of-sight. Perhaps distance figured in there as well. The place that it was easiest to test was on a level for which we don't have the map file. That made it even trickier to pin down the cause.
There were only a few more bug fixes that weren't already in the 1.3a patch, and non of those are serious, so there isn't a compelling reason to create and release a new patch without a fix for the targeting bug. Another reason to put it aside is the stern language in Ubisoft's new EULA. It sounded like thay didn't approve of what we were trying to do, although it's uncertain if they would exercise their right to stop us.
Another reason to put the effort aside is that there are much better game engines to work with these days with very strong support communities. Also, the MM6-8 game engine has been sufficiently hacked that folks can make new games.