Looks interesting. Good to see *something* since the demise of the Dreamcatcher.
In practice, most mummy bags can be opened up and spread over you like an asymmetric blanket, at least in warmer weather.
But you get cold feet if you move more than a nanometre in either direction. I sometimes end up doing weird leg things with a partially-unzipped bag, but that's a bit more complicated to adapt to the changing temperature than a blanket.
With a mummy bag, it's easy enough to keep your feet tucked inside the bag while the rest is opened out. A rectangular one opened out basically is a blanket, albeit probably not quite wide enough and with a potentially annoying zip. I agree a decent blanket taken from your bed at home would be more adaptable but it's also heavier, bulkier and probably not as warm when a hot, sunny day is followed by an unexpected sub-zero night. It's all compromises one way or another.
I'm not quite convinced by the shape of the Nemo Riff, for me personally. It looks a bit lacking in knee room, which is what I'd personally want. But it's good to have another shape available for those who don't quite get on with the conventional mummy, the standard rectangle or a quilt (or a blanket!). A bit like clothes (or saddles and bars) it's an area where variety is good.