Imagine seems to be a straightforward implementation of a stream processor design, heavily indebted to early Cray designs. If your problem is stream-shaped then it should suit your needs. The market for it is graphics and network data-planes, and the competition is GPUs and custom proprietary network chips. It isn’t (and doesn’t claim to be) suitable for general-purpose applications.
The Mill is a general-purpose architecture. While we can do a better job on graphics or stream loads than other GP architectures, we are not suitable for the sort of dedicated heavy loads that one might want a GPU (or Imagine) for. The two architectures are complimentary – I can see a network product with a Mill for the control plane and an Imagine for the data plane