Probably someone from the unleashed project can give a better answer, but my understanding is mostly unhappiness about the illumos contribution process.

The illumos RTI (request to integrate) process is designed to ensure a certain quality of contributions. The downside of this is that it sometimes takes a long time to contribute a change.

For illumos there is also pretty high expectation of backwards compatibility. What unleashed now offers is a place to do more radical changes and quickly iterate on new ideas.

While the small illumos community is already pretty fragmented my personal hope is that in the future work done by both projects keeps getting merged from time to time. Maybe this also serves as a wakeup call to the illumos project to make some improvements to the contribution process itself.