Even some of the bigger models are a huge PIA to paint. A thunder hawk for example, I don't even want to touch mine because I don't own a paint gun and to trying to neatly base coat a model of that size and make it look decent is a challenge and a half. Most of the apocalyspe models seem like they will be just as hard as well due to their size. It would seem that more detail could seem difficult to add as well. It's easy to knife in some bullet holes and battle scars/marring into a small or medium sized vehicle, but something on an huge design? You'd have to figure thats a lot of precision work and time to make it look just right. Having too little or too much would throw it off - or having the markings be too little or too big in size would make it look funny. Its a tough thing to judge when you think about it.
That and I absolutely mega loathe working with resin. Period.
