The idea that the Jedi fell because of their own incompetence, as portrayed in the prequels and fleshed out in Clone Wars.
Also the fact that they unwittingly bring destruction every single time: Yoda trained Dooku, who became a Sith and helped Palps start the Clone Wars; Obi-Wan trained Anakin, who became Vader and wiped out most of the Jedi Order; Luke trained Ben, who became Kylo Ren and wiped out his Jedi academy and burned it down.
Luke thinks they’re inherently flawed as a concept because of this.