Looks cool as a toy, but the advantage of qnap is not the hardware (which is just cots), it's the software. Ain't nobody got time to mess about with mdadm. And no, Webmin isn't even remotely in the same league. I totally love the reliability and ease of use of all the qnaps I had over the years.
You almost never would use mdadm directly since you can just use LVM which uses mdadm under the hood. Still no need for commericial software just for RAID.
I personally use solutions like qnap or synology for my personal use. Not that I dont know how or could not do it. It is more my patience for messing around with disk configs is rather low these days. Years ago I would mess with it endlessly and worry about every bit of the config. It was sort of fun in its own way. Now as long as it stores my files and can max out on my network I am pretty good and willing to pay to get ease of setup. If you are willing to trade time for money then go for it.
