Very nice pics Martin, wouldn't mind seeing one of those in action, there certainly different :)
As for trailed fert spreaders, transport and field speed play big parts I think. 3.5 ton of fert sat on the back of the tractor is stressful enough, but doing 10-15k over a bumpy field or 40-50k down a pot hole ridden road when fully laden must be 10 times as stressful on the arms, axle etc espeically when you hit a bump. I ripped the threads off our top link when fert spreading a few years back after hitting a pot hole in the road, spreader dropped like a sack of tatties, sending fert everywhere including the bonnet of the car behind me. Did I tell him fert would rot his tin work, did I eck :D :D :D