Parts with walls of uniform thickness allow the mold cavity to fill more easily since the molten plastic does not have to be forced through varying restrictions as it fills.
If the walls are not uniform the thin section cools first, then as the thick section cools and shrinks it builds stresses near the boundary area between the two. Because the thin section has already hardened, it doesn’t yield. As the thick section yields, it leads to warping or twisting of the part, which, if severe enough, can cause cracks.
What if you cannot have uniform walls (due to design limitations)?
If design limitations make it impossible to have uniform wall thicknesses, the change in thickness should be as gradual as possible.
Coring is a method where plastic is removed from the thick area, which helps to keep wall sections uniform, eliminating the problem altogether.
Gussets are support structures that can be designed into the part to reduce the possibility of warping.