The weakness of synthetic insulation is that it is never quite as effective as top-quality goose down, which is still considered the gold standard. To get even close in terms of insulation performance per unit weight you have to use some pretty high tech and expensive synthetic constructions and you still fall short.

The weakness of down is that it doesn't handle extended exposure to moisture, chemicals or compaction well. All those are real issues for a sleeping bag that is going to spend a lot of time in a vehicle. Inexpensive synthetics are pretty good around water, chemicals and they can recover from being compacted fairly well.

While synthetics fall short in insulation performance per unit weight weight itself isn't a critical issue for a bag kept in a vehicle. IMO the ability to stand up reasonably well to compaction, moisture (say from a blown window seal) and chemicals makes synthetic the better choice. Better to have something that is a little less efficient but which you can count on than a top-grade unit you can't.