Comprehension!
For Dao Integration, Void Refining, and Nihilism, your main worry was having enough comprehension to understand the natural laws of the world in order to integrate them into your Essence Soul in preparation for becoming an immortal.
This third test was the most crucial to the two sects because their highest level of power was capped at Dao Integration realm, which was even why the Shi Luo Kingdom was seen as the weakest officially recognized territory in the entire human domain.
As for why Dao Integration was the stopping point, Dalvin had no idea at this point but he would definitely find out later. Whatever the case, for the sects to not only surpass each other and dominate the entire region but to break the chain limiting them, they absolutely needed a disciple with exemplary comprehension to carry forward.