I often associate underdeveloped sweetness with a fast drying phase for the reason you mentioned. When a good coffee is leaning towards grainy flavors and just not showing the level of sweetness that I know it should, adding more time to the drying phase often helps. Post 1st-crack time also effects perceived sweetness, of course, but I think most of this is due to reductions in perceived acidity rather than creation of sweetness.
The causes behind underdeveloped aromas seem to be much more variable. For example, in my experience, a lot of powerful Kenyans, including the two I'm roasting right now, need relatively more time after first crack to fully develop the aromas I'm going for (currant, juniper, blackberry, vanilla, etc...), regardless of the overall time. The fruit notes in many dry processed Ethiopians, on the other hand, don't seem to benefit from increased time after first crack. With these coffees, I'm usually more concerned with losing fruity/floral aromatics after 1st.
The portion of the roast that I'm most curious about right now is the time between the yellow stage and first crack. I usually focus my attention on the drying phase and post-1st crack phase, and shape the middle stage so that it allows me to do what I want with the other two.
But, getting back to your question, I agree with Christopher. Underdevelopment can have a lot to do with what is happening after first crack. I think underdevelopment can mean a lot of different things in terms of sweetness, acidity, aroma, etc...