Another point to consider is the vendor side of the equation. There are zillions of merchants that currently accept BTC, but I seriously doubt a significant number of them will start accepting BTC-U for some time to come.
Graphene is not an easy technology to quickly get up to speed on, and there aren't nearly as many tech-heads that are familiar with it as are BTC and it's similar derivatives.
There are few tools to help with integration, no shopping carts or web authentication tools. Steem was wise to address this shortcoming and thus it has on-boarded many devs and tool makers.
So as much in favor of BTC-U as I am, and as genius of an idea that it is, I believe it will take a big effort to convince BTC users to use BTC-U until (or if) the merchants do.
BTW, why are all the graphene chains on ShapeShift unavailable? If any dev team should have the chops to handle a wide range of altcoins it should be them.
ShapeShift is down for maintenance at the moment but the last time I checked bitshares was still listed there.