Unicycling around town will encounter a lot of uneven surfaces or abrupt changes. Wheel size plays a big role in how much of a problem it is, depending on the bump or divot. For instance, I find it difficult to ride a 20 inch freestyle uni in the downhill to uphill transition from a road up to a sidewalk sometimes, especially if it's steep. Move that to a 36er and I barely notice (in part because inertia carries you through). I'd say little cracks are generally unnoticeable on either. Hurray pneumatic tires!
A lot of going over an obstacle (when you're not jumping), is anticipating it and getting the wheel just a little bit in front of you. Once you're comfortable riding a uni and the uni is a little bit ahead of you you'll notice that inertia will catch your back foot and pull you back up onto the uni, I find. So, if you hit an obstacle with some speed but preparing to brake it'll push the wheel up and amplify catching your back foot to pull you up and forward.
If you think about it, if you're ahead of the wheel and you hit an obstacle, you're either falling forward or your shoulder is going to have to pull the unicycle faster than you're going to fall (somehow). This could work if you're also jumping instead of just rolling over the obstacle, though, as you could swing your uni in the jump.
As for drops, you just need to see them and not pedal while you're falling. Again it's more difficult with smaller wheels (since you can hit the deadzone quickly if your wheel is free, and usually short cranks limit your torque), but it calls more for attention than skill. A random 1 inch divot is not hard to ride over, but you'll probably fall off if you don't see it coming and adjust your pedaling cadence (on a small wheel especially, but unexpected falls can make you fall off bigger wheels too). Dropping off a curb is not a big deal if you keep your feet on the pedals and somewhere near neutral or carry some speed.