In the scene where Zhao Yunlan is adorably posing in his new badass outfit, Ma Gui says "Even we can't fully exploit all of the Holy Tools' powers. When Da Qing touched them accidentally, it looked like he was affected somehow." So they don't fully know what the Hallows can do, and it's not exactly a good idea to find out? Unless you find out that they make you basically immortal, then it's a good idea I guess. xD