Anecdata rather than a rigorously conducted survey across UK schools. It was very much the consensus amongst my teacher friends when I kicked off a discussion on the topic whilst attempting to donate a handful of them. (I had a bunch spare from doing some speed testing on them.)

Interestingly, they also felt the microbit had been much more successful and accessible, so I guess it's not a lost cause to create a product for that market.

The foundation do publish yearly reviews (not sure if they done a 2018 one yet) on their website with statistics they have gathered.

But it's interesting the microbit was considered more successful in your group. Perhaps having the name of the BBC behind it gave it more weight. Or the fact they gave them away free to younger children.

I think maybe it's also a simpler device to understand end-to-end, a bit like 8-bit micros were when I was a kid?

I think this is it. It's a simpler device which is easier to get started with. At the cost of being able to do less with it but that's a good tradeoff, in primary school at least.