what were the main problems of Gnosticism from a social/ethical point of view?

I know a bit about their doctrines but what were the main problems when one views the issue more pragmatically?

Well, their belief that they have inside information is problematic for them.

On one hand they sanction elite ism. Or they are liars. Either way being exclusive rather than inclusive is a problem.

It clashed with the main body of the Church, therefore it had to go.

Just as Christ said some Christian Churches incorporated sexual practices and idolatry. I would think some early Gnostics tried to fit their sexual desires into the Teachings
of Gnosticism. Which made the rest seem false.

one of the problems was the sharing of the teachings, men and women, equal, they took turns in the different functions that they preformed, they had no permanent place of worship, no hierarchy's of power and they believed that each being could find his/her true connection to god/creator in this life! And they did not pray to the Jewish god of wrath, but to a more loving form of god and his spouse, as they thought there were two gods. The other followers of Jesus, were male oriented, and wanted a power base and control of their followers, so they formed the church like the Roman government, and ended up with power ,control, and wealth