Talent couldn't be judged easily.
Some people had obvious talents, like Celeste from the Veritas Clan, but not everyone's talent stemmed from a great bloodline, physique, or affinity.
Talent came in many forms, to the point where it was rarer to find someone without any talents than someone with one.
Therefore, Damien didn't make his tests where only the strong could win. It wasn't a place where obvious talents would shine, for those didn't need his light.
The three tests were based on three aspects Damien considered important for troops under him. They didn't have to fit all of them perfectly to be accepted. They didn't even have to pass one. But everything from the way they went about their tests to the order they took them in gave Damien ideas on who fit his requirements.
Hershel's first test was soul.
His body and mind disappeared, and his soul was temporarily erased of all ego.