Astute observation. Kun was arrogant and secure in his power. He doesn't use the amulet blasts against any of the Jedi Masters of renown, instead preferring to destroy Vodo in saber combat (as Vodo was an exceptional duelist) and Odan-Urr with the Force (Odan-Urr being basically the de facto Force sage of his era). While people have indicated that Odan-Urr is too old and weak in the Force to be of merit (something they would never level against Yoda), this argument doesn't hold water. Exar Kun is intended to be beyond the reaches of anyone as DLotS. The comics pretty much nail that point home.
Exar Kun honestly had a lot more up his sleeve than we saw during the comics. This assumption is proved in Dark Apprentice, which shows him using powers he didn't do in his lifetime, and it's again heavily implied we haven't quite seen everything. Though, some examples from Dark Apprentice that he didn't use in the comics are:
-Dark Side Tendrils
-Soul-Ripping Ritual
-TK ability on level to Force Choke 12 Jedi Knights at once
-Heavily implied mastery of Force Illusions