the only issue I have seen with Diagonal links is if they are under sized.. . the longer they get the bigger around they need to be. They will bend if too small a dia on hard leaving cars and in situations where the car gets out of shape.
WIN, what would you consider under size ?
anything less than 1"x.083 (then it better be short). As stated prior as length increases diameter should too. There is a customer car here in the shed that has a .875dia by 44" long that I am going to recommend a larger diameter. The bending moment forces increases dramatically as Tube Dia increases marginally so bigger in this case is better with little to no weight penalty.
The advantage of a wishbone is you can make suspension tune changes (4 link bar position/length) without adjusting the wishbone. Additionally the wishbone can be mounted above the axle center-line allowing for center section/drive shaft drop out ( 8 3/4 or 9") where a diagonal link cant. Also a wishbone has three load points vs two on a diagonal link.
An X link is to a degree an upgrade from a wishbone as it has four load points and it pivots about the center of motion... seems like over kill but they were introduced at a performance level above my experience so there may be some benefit .
