Li Fei was precisely the type of person who sought revenge with patience, and such a person is cooler than anyone else. Because of this, all his choices were made from a cautious standpoint. It was precisely this caution that made him appreciate Feng Tang even more.
After all, if caution was the priority, he should not have chosen Li Fei, especially since, as Feng Tang himself had said, Li Fei had arrived in Jingcheng without any power or demonstrated strength. If Li Fei had some kind of backing, that would be a different story, and Feng Tang might have made such a choice. But even without witnessing these things, the cautious Feng Tang still chose Li Fei.