If you think it's overpowering now, I cannot imagine what being able to pick traits would do... Seriously, the balance would be so far gone it's not even on the same planet..
The problem is that we only remember what frustrates us, if everything works well it doesn't tend stick out in our mind. On the other hand if we have one bad game it sticks out like a sore thumb.
Food for thought:
How many matches do you remember your devour/deathmark going off and helping you in a match?
Now how many matches can you recall where devour/deathmark went off on you and you suffered for it?
I honestly do not believe any of the devour abilities/deathmark are firing off more then they should. The times they do and we get put in a bind because of it just leaves an unpleasant taste in our mouth.
The same can be said for cascades.
I had a match recently where on turn two, the computer proceeded to loop Dragon Soul enough to wipe my ENTIRE team on turn two. I was swearing off a storm and insanely pissed. On the other hand, while I cannot remember the specific instances, I know I have pulled something similar off on the AI numerous times and didn't think anything of it other then AWESOME.