The Fisher is an experimental Delta printer that has a properly rigid frame.
Like the Ultimaker 2, this design uses panels to extremely good effect, providing complete rigidity of the frame. The use of plastic in the corners however means that it is not entirely a triangular frame, so the rigidity provided by the panels will be transferred via a "lever" effect into twisting of the plastic pieces. In reality however this is unlikely to be an issue.
Interestingly an experiment with straws, paper and tape quickly showed a potential design flaw in the Fisher: the upright struts could still potentially "bow", flexing the panels out of shape. This is easily fixed by adding a third (middle) set of horizontal (triangular) bracing (which may have to go on the outside in order not to interfere with the Z-axis mechanisms). The extra bracing ensures that the panels, across the top of the "open" part, do not bow / flex. https://youtu.be/6Fi1tcS3tn0
Here is a quick sketch of a panel that could be cut out and used to create complete rigidity of the Fisher frame. Care needs to be taken to ensure that the panel (which would need to be attached in 9 places just above the opening in each of the upright panels) does not interfere with the Z-axis mechanism. A video demonstration page for the concept is here Mechanical_Rigidity/Video_demo_Fisher_improvement
The Fisher is at heart a really good design.