It is not that difficult to understand that all what is conditioned is liable to cease. All of this nature (sankhata) is not reliable, not stable, not constant, not-desintegrating. It cannot function as refuge, safety, protection we seek amidst this burning, fearful, violent, insecure world.
Also nice feelings, nice mental states, nice existences, are sankhata.In its change, in its arising and its ceasing, it is not stable, it is also unreliable, burning with unsaftey, unprotectedness, and that is dukkha, and not what Buddha sought.
Buddha teaches the Path to the stable, constant, not-desintegrating (Asankhata, SN43). This is reliable.
Not-dukkha. Nibbana is not dukkha. But as long we do not know the stilling of all formation, peace, Nibbana, cessation, which can be directly known (MN1 and others), we are also not really able to understand the First Noble Truth. For example, how can one know that all feelings are dukkha, if one does not know that the cessation of all feeling states is bliss? I believe Sariputta did have that knowledge. He directly knew that the cessation of perception and feeling is bliss. And then one also knows that the re-occurance of perceptions and feelings represents a modicum of stress.
‘Here there is no stress due to the defilements of sensuality, desire to be reborn, or ignorance. There is only this modicum of stress, namely that associated with the six sense fields dependent on this body and conditioned by life.
The idea that we must not take it literally that formations are dukkha is by some felt as a judgement i think. A standpoint. But i believe that this only show that these people do not know Nibbana, like for example Sariputta did.
In the end, we do not know even the first noble truth when we do not know Nibbana. We will never be able to understand why a noble knows that even nice feelings are dukkha, if we have not the reference of the bliss of stilling of all formations, Nibbana. I believe.
From knowing the stilling of all formation as blissful one also know: that only suffering will cease.