hmm for things I would not want to draw, it would be more based on the subject matter to me. For complicated stuff, I actually try putting that in my comic a lot so I can be forced out of my comfort zone. (for instance, car interiors.. I really hate them, so I made myself draw them a lot) but if life is getting in the way and such, I end up cutting back on the details of backgrounds since they tend to take up a lot of time for me.
Other current weaknesses are spiral staircases, mechanical parts, just interiors in general, the list can go on xD;;