Can i start by pointing out that everyone who has slung a leg over my own early stumpjumper completely agrees that it is awful- one person remarked that 'It's a real wonder how these things caught on'.
Also remember that they were built down to a price- they were almost a copy of the early Ritchey/Mountain Bikes that messrs Kelly and Fisher were selling at twice the price (and a long waiting list).
They are pretty crude and probably represent the level of technology available in Japan at that time- they were new at this afterall. The alignment is relativly poor (and i might add that mine is almost NOS so its not a tired example), it is very heavy and the geometry and fork rake is all over the place resulting in a bike that does not like slow speed turns and shimmies and frets in high speed turns. It doesn't like climbing nor descending and bounces all over the place on anything other than smooth trails. So, apart from cruising down to the shops, its a pretty poor bike by any standards.
I did read somewhere that when Mike Sinyard approached Fisher regarding the purchase of two Fisher/Kelly's, that he had an idea that mike was going to rip off the design, so they supplied an incorrectly specced frame and fork that threw out the handling somewhat. A few remaining frames from that batch have recently turned up NOS in a bike shop in Sausalito where they have rightly remained unsold for 30 odd years.
So, the stumpjumper was flawed before it was even born. It would go some way to explaining just how badly they ride.