One way I think about saṅkhāra dukkhā is that,
Because of Aniccā nature of all saṅkhata,
We always have to engage in doing saṅkhāra of various types until PARINIBBANA.
Now, when we do saṅkhāra for saṅkhata with Aniccā nature,
They keep changing, so we have to kee putting efforts without any end in sight.
If saṅkhata worked as per iccha/will/desire/wish,
There would be no need to keep doing saṅkhāra.
But, because they don’t,
We have saṅkhāra dukkhā.
Another aspect doing saṅkhāra for any saṅkhāra is like embracing feces.
Because they do not ever lead to anything substantial.
Despite putting in even unlimited amounts of effort for Anything in this world,
It inevitably is lost, decayed, perished.
So, doing saṅkhāra for saṅkhata with viparinama nature makes no logical sense.
This is how I understand saṅkhāra and viparinama dukkhā.