Sorry I didn't see this earlier. I don't have a specific list of mods to avoid using, but in general, custom campaigns work best with a small number of active mods -- a few dozen at most.

Left 4 Dead 2 can only use 4 GB of RAM no matter what, so the quantity of mods (particularly the quantity of changes they make) has the biggest impact. For example, mods that replace the models and textures of every zombie are problematic, as are HD texture packs.

I sometimes get a crash near the beginning of Map 1, when you climb the humvee to get on top of the Semi trailer (where there's usually a magnum) with nothing else on except improved bots mod. Not sure if the mod's causing it, but if it does crash, that is consistently the place. Hasn't happened anywhere else, which is weird consider how much busier other areas of the campaign are.