The underlying platform services seem to be pretty solid, but I'm disappointed in the quality and usability of the new UI.
I'd argue that it's a start. Developing a new UI is always going to be an iterative process. I don't think the developers have yet to finalize what they want, UI-wise, so there are aspects that don't quite make sense. Let me give you an example:
A long time ago, I spent way too much time developing a web site that is largely hand-coded. One of the decisions I had to make was how to present the content. Would it expand and contract or is it allowed a maximum width, and no more? The latter approach is arguably more pure (let the user decide!) and yet aesthetically it didn't make sense. Nor was it readable. So I set a minimum and a maximum width. Bigger browser windows would hence be filled with plain background.
But deciding on such maximums and minimums is important, especially in the context of a complex UI. Yes, a completely dynamic browser experience is possible (let the user decide!) but it makes it far more complex to develop a UI that is usable in all circumstances. Some things need to be compact and shouldn't be spread across the window.
There are windows in the new FreeNAS UI (like the reports page) that implicitly rely on the user having a 4K screen. For example, the hard drive reporting page pretty much needs a grid view whereas other windows (like the pools view) become quasi un-usable IMO when the window is large enough to accommodate the hard drive disk temperature grid reporting view because the data is so spread out that you can't easily see which HDD just failed and needs to be taken off-line before being pulled. The data is simply too spread out.
Resizing the browser window brings some relief, but ultimately those kinds of data-table windows should be just wide enough to accommodate the data therein and no wider, so that it's easy to follow the information in the data table from left to right. If there is "empty" space to the right of the table, so what? I'd even argue for giving users the option to enable an old feature in the Dungeons and Dragons books where every cluster of three rows in a table is either the background color or a slightly different one to help your eye stay in the proper row.
However, the best solution could be simpler where the command/control UI sticks to one window target (built around a 1080 screen, for example) while accommodating the "larger" reporting views by popping up a separate window in which to show that data. That's how I would like to look at my FreeNAS when I'm using it - one compact UI window for commands, tables, and so on, and a separate set of browser windows for hard drive temperatures, CPU temps / tasks, and console messages.
That way, your mouse also doesn't need to constantly skate vast distances to get to menu options. This is another aspect of the new UI that I shake my head at. The UI should not be built around the idea that we all use touch-screen computers. For example, I don't understand why in the storage view the pool is expanded into view on the left side of the window but all commands associated with the pool (detach, etc) are done on the opposite side. Every time you do something to the pool, the pool view snaps shut like a clam and has to be re-expanded into view. Why?