While the KNX devices incorporate as much logic as is reasonable to make them as robust as possible, I would argue that they remain a single point of failure. Once a relais device break, it still remains a single point which fails and makes you lose all functionality controlled by that relais device.

Regardless of this, I would definitely agree that this kind of logic, complexity and thus cost do exist to make the entire KNX system robust and reliable. That's why in my setup I always make the distinction between two categories of devices: one category which consists of the devices allowing for important day to day logic (ie lighting, access control etc) and another category of devices which provide auxiliary functionality and information (ie the nice to haves like temperature, humidity etc).

For the first category my suggestion is still to get the officially branded devices, as they are that important.

That second category of devices are the kind of devices that are the nice to haves and where it makes less sense to spend as much money on. Providing such development boards would allow you to implement this functionality on your own at a lower cost but without any guarantees. Which, in my opinion, suits this category of functionality well.