I run FreeBSD ZFS on my desktop and have for years, and we're thinking of using it (for the root fs) at work. I considered the ZOL integration to be good news. We'll get better CI, and we'll share features with ZOL faster. I don't really see the downside.
I've also run ZOL for 7 years on my wife's desktops..
My understanding from the message, and from talking to the devs involved is that integration with ZOL will make FreeBSD an equal partner. FreeBSD will be included in the CI, so that ZOL changes that break FreeBSD cannot be merged. Changes originating in FreeBSD will also be merged faster to Linux, and with review from the original author to make sure they are merged correctly. Eg, it is better for everybody (except perhaps the folks left on Illumos or other Open Solaris forks).
At first came the interesting question of "what is OpenZFS"? A spec? A reference implementation that everyone branches off? How do we exchange fixes and ideas and updates?
Another topic that came up was "What should the defaults for a zpool be?" this was for portable zpools.
Please note: the above were still open questions. We're going to have a few follow up calls to understand and ideate better.
Source: I was on the call, and I'm an Illumos community member.
[0] https://docs.google.com/document/d/1w2jv2XVYFmBVvG1EGf-9A5HB...
ZoL may have more development activity behind it, but due to licencing, it will never be a first-class citizen in Linux like it is on FreeBSD/Illumos. That limitation makes the decision to turn OpenZFS (essentially, ZoL) into the upstream seem unwise.