Hi finnaeus,
You can see my thoughts about flexible slat support systems in post #4 here and the posts it links to.
While they wouldn’t normally be my first choice for most mattresses they can be a good choice for some people that would benefit from the additional flex under a specific mattress and if they have tested a specific mattress on a flexible slat system and it’s a better match for them in terms of PPP than the same mattress on a firm non flexing foundation then they can would make a suitable choice.
They are often used in Europe underneath thinner mattresses as an “active” component in a sleeping system.
Phoenix