Titans are great at PvE but suck at PvP because their abilities favor running into the middle of a battle and either getting massive kills or protecting endangered players. You can't get those massive kills in crucible because some players can one hit kill you with abilities like golden gun. You can only protect.
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I get massive kills with my hunter, though. Hunter's abilities favor quick one to two hit kills, stealth, and evasion while warlock's abilities favor spamming grenades, blowing groups of players up with nova bomb, and teleporting to evade fire. Titans are o.k. with taking damage and great with defending players.