As noted, there is an invaluable wealth of off-road tandem experience on this (and other) forums. That experience has shown, repeatedly, that certain geometry, components, materials and accommodations make off-road tandeming easier and more fun.
Frame material choices have changed over the years to the lightest, stiffest platform, since suspension and tires do so much more to handle bumps than frame material ever can. Thus, alloy frames on average-priced stuff and carbon on high-end stuff. Just like with single bikes, tastes have changed and new materials work better. Since tandems place some unique stresses on materials, lateral stiffness is even more important, especially off-road where laser-sharp handling is sometimes critical to a successful line. That's why most tandem frames within the mainstream enthusiast market are alloy, with carbon on the high end.
From my position, there is some frustration with both Cannondale and Salsa because they keep making (what I consider to be) the same mistakes - inappropriate geometry for true off-roading. In the case of Salsa, they also have chosen a less-efficient frame material, and on top of that, have spec'd the tandem with parts that will likely not hold up to off-roading (or loaded touring, which appears to be the intent of this design).
It's not like any of this info is secret or anything. These forums are full of the experiences of lots of teams who have learned expensive lessons about what works and what doesn't. A few days spent here would probably save a company a great deal in warranty costs and ill-will. In fact, when Salsa first "leaked" info on this tandem project a few years ago, the feedback they received (many from Salsa fans, since it was leaked on a Salsa forum) mirrored the comments that have been made here since the introduction.
Since they haven't chosen to pay attention to the experiences of riders who have "been there, done that", one could assume that they either are not very bright (very unlikely - Salsa's a successful brand), or their intended use is not what we think it is. Maybe the best analogy is SUV's; very few people actually use them as they were originally intended, and as a result current designs are not nearly as capable and durable as older ones (electronic aids notwithstanding). but a lot of folks buy that "look". Truthfully, it seems to me that the geometry on this tandem leans more towards gravel grinding, and perhaps dirt touring, not full-on mtb tandeming. From that aspect, I think the Co-Motion Java might be more the intended target of this Salsa tandem, not a Fandango or a Ventana. This is a Tandem Mountain Bike forum, not a tandem touring forum, so one might expect some criticism from the perspective of the members here.
Salsa will sell these to people who buy Salsa, just like Cannondale did and will do for a bit longer, at least until their US-brand cachet finally wears off and people realize it's another Asian-sourced import. Does where a product is sourced from matter? It does to me, it doesn't to some people.
I sell most every brand of off-road tandem (or wannabe) made; I'm not too proud - I'm a tandem dealer, after all - if the Salsa is available to me, I'll sell it as well. I'm also very up-front with customers about what a tandem will and won't be good at. And what bothers me most is that some folks will buy these tandems, have an experience that is less than it could have been, and in many cases choose not continue to ride off-road, thinking it's not practical or feasible. And that is lost business and lost opportunity for our niche of the sport.
Perhaps it's unrealistic to expect everyone here to agree with or support what seems to be an assertion that a KHS with a suspension fork and wheelset upgrade, the new Salsa, The Cannondale T29, or similar products, represent a good off-road tandem platform in the wider context of what's available now, and especially within the context of the style of riding many on this forum participate in. From the point of view of many on this forum, the new products we're seeing aren't even as good as what's already available for what we use them for. Like all enthusiasts, we look forward to new and better products being introduced.
The members here speak from their own and shared experiences, and those experiences are very valid (and valuable).