I have bought Qnap TR-004 recently to use it as simple external enclosure with disks set in individual mode for a linux server. In other words: I had all disks visible separately in the system.
Unfortunately I had to return it for 2 reasons, important for my needs:

1. Inserting or removing a disk interrupts the USB connection with the computer

This means if you create, let's say, a RAID 5 under linux and you want to replace one disk from it, you will have to unmount the filesystems using the raid and shut down the software raid. That is not productive at all.
Another customer with the same problem explained it here: https://forum.qnap.com/viewtopic.php?f=24&t=158465&p=778616#p778616
I have also uploaded a video on youtube explaining this problem:

Check out my channel for other interesting videos.

2. The hardware SATA controller maximum speed is 3.0 Gb/s

It supports of course SATA III disks but they will run only at 3.0 Gb/s.
I have discovered this from smartctl output:

root@minivm:~# smartctl -i -d qnaptr,0 /dev/sdc | grep ^SATA
SATA Version is:  SATA 3.1, 6.0 Gb/s (current: 3.0 Gb/s)

That is confirmed in qnap forums: https://forum.qnap.com/viewtopic.php?t=145628

Other than that it is a fine product and I am sure useful to the right user.
I did mentioned in the title: short review :)

As bonus, S.M.A.R.T. information is not available out of the box on linux.
If you try to do this with smartmontools that comes with the OS, you will get this:

root@minivm:~# smartctl -i /dev/sdc
smartctl 7.2 (build date Jan 29 2021) [x86_64-linux-4.19.0-13-amd64] (local build)
Copyright (C) 2002-20, Bruce Allen, Christian Franke, www.smartmontools.org

/dev/sdc: Unknown USB bridge [0x1c04:0x0013 (0x6111)]
Please specify device type with the -d option.

Use smartctl -h to get a usage summary

To enable such useful feature, especially if you use it with software raid, you will have to:

1. download and compile smartmontools

from here: https://github.com/slade87/smartmontools

2. Run it with the following extra arguments:

-d qnaptr,0
where 0 = the position of the disk in the enclosure.
Or disk number as you can get from the output of fdisk command:

root@minivm:~# fdisk -l /dev/sd? | grep -B1 TR
Disk /dev/sdc: 3.7 TiB, 4000787030016 bytes, 7814037168 sectors
Disk model: TR-004 DISK00   
--
Disk /dev/sdd: 3.7 TiB, 4000787030016 bytes, 7814037168 sectors
Disk model: TR-004 DISK01   
--
Disk /dev/sdd: 3.7 TiB, 4000787030016 bytes, 7814037168 sectors
Disk model: TR-004 DISK02   
--
Disk /dev/sdd: 3.7 TiB, 4000787030016 bytes, 7814037168 sectors
Disk model: TR-004 DISK03

So if you want to get smart information from /dev/sdc, you would run:

root@minivm:~# smartctl -i -d qnaptr,0 /dev/sdc
smartctl 7.2 (build date Jan 29 2021) [x86_64-linux-4.19.0-13-amd64] (local build)
Copyright (C) 2002-20, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Model Family:     Seagate IronWolf
Device Model:     ST4000VN008-2DR166
Serial Number:    XXXXXXX
LU WWN Device Id: 5 010s30 0x3q3r544
Firmware Version: SC60
User Capacity:    4,000,787,030,016 bytes [4.00 TB]
Sector Sizes:     512 bytes logical, 4096 bytes physical
Rotation Rate:    5980 rpm
Form Factor:      3.5 inches
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   ACS-3 T13/2161-D revision 5
SATA Version is:  SATA 3.1, 6.0 Gb/s (current: 3.0 Gb/s)
Local Time is:    Sat Jan 30 02:19:36 2021 EET
SMART support is: Available - device has SMART capability.
SMART support is: Enabled