By embark, I mean when I hit E on the world map; I'm not even getting the chance to select stuff to take. I don't think it's based on items I'm taking, either, because I have a successful working save in another file, and all my modded items appear to be working fine. Migrants and caravans appear without causing the game to crash in that save, as well, but it randomly decides to die after about half an hour of play.
EDIT: After some testing, apparently I can embark onto sites from my custom worldgen (mostly desert, grassland and badlands) but I can't embark into areas from the standard worldgen. This confuses me greatly. I'll try removing my modded files and sticking them back in one at a time until something breaks.
EDIT EDIT: This is freaking strange. It's crashing at random, apparently, because it will sometimes allow me to embark onto a site and at others times it crashes, for the EXACT SAME SITE.
EDIT EDIT EDIT: Interesting! Up until the point where I deleted matgloss_plants from my fresh install it was embarking fine. Are some vanilla plants mandatory? Editdoubleplus: adding matgloss_plant back in from my DF zip appears to have fixed the crash issue.