I do think Titans are at a bit of a disadvantage (especially in pvp) but not one so large that you cannot do well with a Titan. I have played all the subclasses as well, and even though the other classes are better in general, Titans feel really fun to play. Though I must say, everyone is so infatuated with Bubble for PvE, I think a Striker can be just as effective. With their signature blend of health and useful aoe attacks, if you can learn how to maintain a fairly low health threshold while not dying and juggling mobs, you can successfully attract and tank many of the attacks from regular adds, leaving your teammates fairly free to take down the boss. The Key to playing Striker in PvE is in knowing just how much damage you can take before needing to get out of dodge.