Leaf springs have been used like this forever it feels like now. Take a look at the lear spring that was used on the rear of the Corvette through the C6. Silly journalists use to say condescending things about this, but of course they had no clue what they were talking about.
It’s a clever use with updated materials, but it’s just a smart application of something that people have been doing for a century.
As for either design being more prone to understeer, I agree with that it’s the car concept its on and not due to underlying issues between the rod orientation.