From what I recall they had a line of higher quality parts that were still much cheaper than the boutique CNC brands. A tough sell, which is why they now have the separate TRP brand for higher end parts.
A friend had the shaft from a USE Ti post fail in a bad way way back in the day, he was lucky to get away with a cut and ripped pair of shorts. It could have been much much worse.
The DH has a separate cartridge for compression and rebound. They were 80mm travel only so maybe there was no need for something like a neutral shaft to limit travel.
Sorry I can't be more help, I'm sure others can add more.
Cassettes are similar construction and spacing, just different gear ratios. I used to run 9 speed Ultegra and Dura Ace 12-27 cassettes on my weight weenie bikes.