I think we get too many NUC requests, some wanting to use USB attached storage. These can be successful NASes, but when things go wrong, they tend to be bad. Not a simple fix. There have been just too many horror stories or problems to consider a NUC box or USB attached storage as a reliable TrueNAS server.
While my media server IS a miniature x64 computer, with 12 year old tech, (4 core, single thread per core, AMD64 low power CPU), it does have 1 x mSATA & 1 x 2.5" SATA drives. Plus, even though it has only 1 memory channel, it can use 2 SO-DIMMs for a whopping 16GB of memory. (Their are reports that 32GB is achievable…)
However, I don’t run TrueNAS. But, I do run Linux with OpenZFS quite successfully. And have for 10 years now. (I really need to see about replacing it… Perhaps a nice DeskMeet X600 is in my future.)
As to WHY NUCs, (even mine), are not recommended, is storage ports. Even if a user has only 2 SATA and uses USB, (with SATA or NVMe adapter), for boot, it still may be too limiting. Thus, someone wants to "share" a storage device.
For example, my media server has this on both mSATA & 2.5" SATA drives:
Device Start End Sectors Size Type
/dev/sda1 2048 10239 8192 4M BIOS boot
/dev/sda2 10240 206847 196608 96M EFI System
/dev/sda3 206848 616447 409600 200M Linux filesystem
/dev/sda4 616448 25782271 25165824 12G Linux swap
/dev/sda5 25782272 84502527 58720256 28G FreeBSD ZFS
/dev/sda6 84502528 3907029134 3822526607 1.8T FreeBSD ZFS
Partitions 2, 3 & 4 are MD-RAID Mirrored, with partition 5 ZFS Mirrored. And the last, partition 6, striped for Media, (which has multiple backups).
This amount of sharing a storage device in TrueNAS is a recipe for disaster. Add in USB, and we are headed for a Titanic style sinking.