Whenever you put a lot of force on a single pedal, you are applying a force to the surface on which the uni is resting, through the pedal, crank, hub, then spokes, rim and eventually the tire. For example, you do a stillstand on a slanted surface facing downhill. There is more force on the rear pedal to counter the forward push of the slanted surface, otherwise you would move downhill. This involves a torque, or twisting force on the hub which eventually puts the backward force on the tire that exactly counters the forward push.
On a radially-spoked wheel, there are no spokes that go that direction (they all go straight out from the hub); so the way the force is transmitted to the rim from the hub is by “winding up” the hub and spokes until the increased tension in the spokes matches the forward thrust. This involves a lot of spoke motion, both stretch and motion within the hub holes. The motion in the holes is called “fretting” and will lead quickly to spoke failure at the spot where the spoke abrades. On wheels spoked with a higher pattern, part of the spoke tension is in the direction of that twisting force so there is a lot less motion of the hub relative to the rim, and thus much less spoke motion in the hub holes.
On unicycles, there is always this kind of torque. Idling is constant acceleration/deceleration. For trials, this is especially so because you may be landing on a curved or narrow surface with a lot of forward and downward momentum to cancel. So you need a lot of strength in that direction because the torques will be high. These torques are much higher, I believe, than those involved with normal riding acceleration and deceleration.
On that front bicycle wheel for road riding with rim brakes, there is essentially no torque passed from the hub to the rim, so there is no need for the extra strength in that direction. Thus one can make the wheel lighter without compromise. For this wheel, the lateral forces are minor as well, so the rim can be lighter and there can be fewer spokes.
For a trials wheel, you just want it to be as strong as possible in all directions. When your technique gets really really good, think about lightening up your uni, but lighten the wheel last.
At Unatics we met a pro trials bicyclist who had a very light bike. He said he could have it that light because his technique was very good. A normal rider would simply break it.
One of the reasons we often use washers under the spoke heads of a uni is to support the spoke head better; this also helps reduce the fretting mentioned above.
These are just generalities but there isn’t much that I’ve found so far beyond Jobst Brandt’s reference. He doesn’t really treat lateral forces and rim stiffness that thoroughly because he is primarily concerned, I think, with road bikes. It’s a fascinating read, though.
Gerd Schraner’s book is very good too but is also focused on road bikes.
Hope that helps.