My experience is the same as Reed and Sharon White. In pretty much every case, steeplechase makes an upper level horse better. It makes them bold, breathe fire (or release some steam!), it gets them so “in the zone” for XC in a way that 4 or 5 warmup fences in a 2 acre field just can’t do. It doesn’t make them careless; IME, they get sharper, dialed in, in front of your leg in a way you can’t imagine until you sit on your horse in phase D. It hones your own eye, gets you riding FORWARD; it allows you to make a mistake, but a “miss” on steeplechase teaches you something without severe penalty. Whoops-- kicked too much there, needed to wait. Wow…that was a long one…better keep my shoulders up and take a breath. You have 6-8 fences to figure it out, without yanking your horse’s face and interrupting rhythm.
That being said… I’m not signing up to take my novice horses to a N3D. The jumps won’t be big enough, the speed not fast enough, to have the same level of benefit for the horse (it still may be beneficial to riders though). A talented youngster will probably run through a novice sized chase fence (vs holding off a 3’9" brush) and not have a true gallop rhythm as an old school P3D at 640mpm.