As I understand it a truss works efficently because as force is transmitted through it one part is in compression and another in tension which leads to a stable shape. A triangle is a stable shape but a square isn't. When you diagonally brace a square you are in effect creating two triangles that form a compound truss which is why diagonal bracing works so effectively.

In the arrangement I posted, the struts should be able to cope with forces in any direction encountered by a cnc machine a lot more effectively than a design based on one or two beams that are fixed to a thick plate either end, or so I thought.

I've just had a quick look around the web for stage collapses and I can't find any where the investigation blamed a failure of the truss system for the accident, do you have any links to demonstrate what you mean?

For example if you read

Indiana State Fair stage collapse - Wikipedia, the free encyclopedia

The failure ... was due to the inadequate capacity of the lateral load resisting system, which was comprised of guy lines connected to concrete "Jersey barrier" ballast. The concrete barriers used as anchors for the guy lines were not fixed in place; they resisted loading only by friction with the ground and through their own weight—about 4,200 pounds (1,900 kg).[6] Just before the collapse, wind loading caused several of the barriers to slide or pivot from their original positions, allowing the top of the truss structure to lean toward the crowd. The subsequent bending forces within the support columns were too large, and the structure collapsed under its own weight.[6] Measurements indicated that the total weight was 70,000 pounds (32,000 kg).