When it comes to a fileserver, I still prefer Truenas.
I've freenas/Truenas for 10 or so years now and unraid for about 5. For the last year I've been working on migrating everything back to Truenas (scale in my case)
Some of my pain points with unraid:
disk read speeds. (Since read is only ever happening from a single disk, it's much easier to notice bottlenecks)
disk replacement. When a disk fails, I find the process of replacing the disk (or decideding to not replace the disk and scatter the data across the remaining disks) fairly tricky and honestly a little scary. I've had to do it twice now and it's the biggest reason I'm now only using unraid to run services but not store any important data.
cache disks are meh. Over the years I've had 3 or 4 times where the mover just stopped, which resulted in a cache disks filling and not flushing to HDDs, which then corrupted some database or file an application was using. Like on one hand you have to use SSD cache disks to run apps or VMs since there is no way to speed up read speeds on HDDs, but on the other it just doesn't work well given enough time.
Some pros:
Application/service hosting is still great in unraid. It's still a pain in the ass getting a VM running on Truenas scale, but with Truenas Scale you can run docker directly.
being able to just add single disks at a time in unraid is nice (until you need to replace one...)
Anyway that's my off the top of my head reasoning. Truenas is a little more work to use overall, but I've found it much more stable
Oh man this is nice. I've been juggling space with an 8TB drive for years (got one when it was the biggest you could get). Recently after deleting some old stuff to free up space I discovered all the newer stuff was fragmented to shit. I was able to squeeze as much as I could onto SSDs and SD cards, defragment the entire thing with Defraggler, and move everything back into defragmented free space. I've managed to pack it like a can of sardines - I've gotten it down to less than 2GB free with no more than 2 or 3 fragments for most files, 7 tops. Still transfers both read and write at around 80MB/s average.
I've experimented with Plex, but since I used to only use the one system with Kodi I never bothered. I recently got a small HTPC to use with an old 3D TV and it's super easy to move Kodi's db to an external MySQL server to sync paused playback and completed lists etc.
I can also just connect Kodi to nVidia shield/moonlight and watch my entire library on my handheld wherever I go, without any additional setup other than Kodi on my desktop and making use of Moonlight which I use for emulating games a bit too powerful for my handheld