I think the misconception is that titans are indestructible/unbeatable, but they are just another unit that fills a roll and MUST be properly supported or you will loose a huge points investment. Different titans fill different rolls and learning proper tactics will determine how useful they are (just like any unit). I use my Reaver as a long range firing platform, and generally don't move it at all during the game. I park it the very furthest corner so that nothing can get behind it and then blob layers of cultist in front of it so that it can't be assaulted. With 4 void shields and 18hp it is a task to take it down with fire at range.
My knights have been very poor performers, in general I feel and are not worth the points to me at this point. This is admittedly because I have yet to find an effective roll for them and they die to easily for such a large point investment. I feel like if I was playing against them they wouldn't be effective in apoc as they would be the perfect target for my Reaver. Large points cost and pretty much a guaranteed kill with one turn of it's shooting with one Laser blaster(thought process is, large enough it likely won't scatter off, min 1hp + 1-3 more hp x3 shots = min 6 hp).
I'm certainly not saying they aren't a worth while unit, I'm just curious how you would use them to get a good return on investment as I have yet to do so.