In terms of positive and negatives...I feel there are loads of positives, and I can see that many people have already raised those..
A negative for me is that I really, really struggle with time management. I believe relationships require a significant (and the definition of significant will be different for each relationship) investment of time. I struggle with finding the time needed to feed multiple relationships to a level where they are given the room and space to flourish and to find their strength.
I'm certainly guilty of packing too much (or maybe just a lot) into my life from time to time...it's often the case that something has to give...and then I struggle to work out what's best to let slip.
Sometimes I feel I simply don't have the time to achieve all the things I want to achieve...and to maintain relationships to the level I need/want. I'm finding this to be the biggest problem for me...and it is the biggest factor in me doubting poly for myself. So, that's the drawback for me - a practical issue..