Have I seen a Nimbus trials ?
No, I haven’t, I really haven’t met many local riders. I read about Nimbus troubles here, in this thread.
High end motorcycle frames are still made from both chrome moly, and alum. It hasn’t been decided yet which is better, but alum is slowly taking over. I think all the motocross bikes are alum, as well as the road racers. Could be a marketing fad though.
Cost spent making it is a better indicator of strength- quality. For instance, the Nimbus axle is solid not for improved strength, but to save the cost in tooling and labor it would have taken to drill it out. The metal in the center of a rod adds little strength, but plenty of weight.
So a reasonable comparison of the cheaper Nimbus to the KH, would show that the nimbus is heavier then the KH, due to a variety of economies taken in it’s manufacture, not because it is stronger. It costs about 1/5 the price of a KH to produce, so there is less work done on it to make it both light and strong.
In any event, I’m not bashing Nimbus. I just don’t see why someone who was going to pay a few hundred dollars on carbon and mag parts to save a pound or so, wouldn’t want to pay 150 $ more for a higher quality uni to start with, that will always be a couple of pounds lighter then a Nimbus. Maybe it would be more fun to put the fancy light parts on a KH. 
But sure, if you can only spend 250 $, the Nimbus isn’t so bad. By the time you add a KH seat and cranks though, it costs only about 50 $ less then a KH 20 from AEB. So you can save 2 lbs. for 50 $ going with KH, but instead spend 100 $ on a carbon seat base that saves a few ounces ? 
I am sure the carbon seat is a fine item. I’m glad I bought a KH 20 when the getting was good (363 $ delivered from AEB last May). Maybe someday I will buy a new seat base, post etc. But I haven’t broke a thing yet, and I’m a bit over 190 lbs.
I think someone who is going to spend 400 + on a trials uni, should get a KH or Koxx one. Or maybe the uber cool Sponge-Ti thing, if that pans out. Why save small dollars by buying a heavy frame-hub if you can afford better ?