I understand this, I myself always get Icebreaker and multi tool, for example last weeks night fall 2 out of my 3 runs I got the MIDA Multitool. since launch I have gotten probly about 12 of those and 7 or 8 icebreakers none from Xur. Although I do have every exotic now including necrochasm a few exotics such as Thunderlord I really had to grind for, played night fall and raid with all 3 characters since mid october and did not get thundelord in till about a week ago. I do not think they make it impossible for certain exotics to drop I just feel that the drop rate for some exotics are higher/lower from player to player.