What trade-offs are being made in the design of the planform alignment of stealth aircraft?
To understand the functioning of planform alignment in befitting the fifth generation air-crafts in increasing their stealth, one first need to understand how Radars work:


Radar transmits a radio wave signal.
This signal when comes in contact with a surface (of an aircraft), it is reflected back. This reflection is a mixture of Specular and Diffused reflection. 
The radar then after receiving the reflected signal, measures the speed (by computing the shift in the time period taken by the wave to be received over a period of time), and size (by computing the intensity of the reflected wave) of the air-craft.


But there is a problem, since the radar needs to get back its own transmitted wave back, if it is reflected off the surface of the plane only at a specific angle (specular reflection), then the receiver of the radar needs to be at specific places to receive the reflected wave. 

Modern planes utilize this shortcoming of the radar by letting the incoming radar signal pass through them (using plan-form alignment), which then greatly reduces the reflected signal. The reduced reflected signal would be seen as an incoming signal of very low strength (intensity) by the radar, which then reads the size of the incoming air-craft as a very small object. This is called Stealth. The air-crafts also use special paints and composite coverings on the surface to again reduce the intensity of the reflected wave, to reduce the percieved size of the air-craft by the radar.

The shortcomings of such radars are removed by using radars such as AWACS, which is moving radar and can receive too directional signals (Specular reflection). Or, using radars such as AESA, which scans a large portion of area around it without having any moving parts.