The troubling part was that even Earth Grade beasts were comparable to third grade humans. Following this trend, one can imagine how powerful a race of Heaven Grade beasts were being of the second grade.
Of course, there were some exceptions to this. For example, Kitsune who were classified as supreme beasts could even sometimes be weaker than some Earth grade beasts depending on their tail count. Dragons were the same, however this was dependent on their Dragon Soul.
Simply put, transcendent and supreme beast grades represented a ceiling of ability, not an absolute ability. This was an equivalent exchange made for them being more human-like and having the human path available to them.
However, Earth Grade and Heaven Grade beasts represented this absolute level of ability, making them highly dangerous, especially in large quantities.
So, the question remained, how could such powerful species agree to be partners with humans so readily?