I think the main issue is the extremes of the environment where the pen is stored -- compared to the environment where it is assembled and finished. If the pen is maintained in a controlled climate (temperature and humidity) equal to where it was finished/assembled, then you can expected to not have many issues even with unstabilized materials. Obviously this is unrealistic, so the question is "how much is the environment going to change", and "how fast". As Tom notes -- oily woods tend to be a little more stable, but they also tend be be harder which can make them brittle, a bit of a double-edge sword. Grain is also something to pay attention to, wood swells and shrinks across the grain but much less with the grain. Cross-cut or angle cut can be more susceptible to cracking. Also, segmenting can cause issues -- different materials expanding and contracting at different rates and in different directions. Finally the finish that you use can be a factor -- more flexible finishes are less prone to cracking.
I had a couple of pens crack early on in my penturning and it was pretty disheartening. I now try to stick with stabilized wood, but it isn't always possible or practical. I haven't had much trouble with straight grain woods that are not stabilized, but I try to keep the blanks indoors and only in the shop while I am actually working on them. Also, I try to warn people not to store their pen where conditions change rapidly -- hot cars for example. That said, I don't sell my pens, so if a giveaway has cracked I probably never will hear about it.