Adstal and jboogscott have shared great insight.
I would also add that if a waterfall of traditional networks, alongside the bidders, was neglected; the latter would adapt and I would not be surprised to see lower bids/revenues over time.
Google Open Bidding has it's pros; which is the plethora of ad exchanges and SSPs connected, so no shortage of demand. However, traditional networks sit outside of this -echoing the hybrid setup which has been shared previously.
I suppose the question boils down to; Is In-App Bidding for you? To keep is simple; if you have the time, resources, and scale, then it's worth considering.