I have had 5 or 6 Legendary Engrams from missions. I loved the fact that I got these! However, excitement quickly turned to disappointment each and every time....as they were all for warlock or titan class...and I am a hunter. I could see it if you got one from killing an enemy, but this was my engram for completely the Daily Heroic at Level 28, my reward.....so why give me something I can't ever use!!! That's like seeing a basket of fruit on the table..and then going to eat it and it's fake.
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Before you redeem a Legendary or Exotic Engram, make sure to hover over it and it will list the possible outcomes. "Random Class" is listed underneath it. If you happen to get something that's not for your class, you can either vault it for a different character later, or simply dismantle it for materials.