The real trick, and the reason I don't build my own NAS, is standby power usage. How much wattage will a self built Linux box draw when it's not being used? It's not easy to figure out, and it's not easy to build a NAS optimized for this.
Whereas Synology or other NAS manufacturers can tell me these numbers exactly and people have reviewed the hardware and tested it.
There are power meters like KWS-303L that will tell you how much manufacturers lie with their numbers.
For example my ancient tplink TL-WR842N router eats 15W standby or no, while my main box, fans, backlight, gpu, hdds and stuff -- about 80W idle.
Looking at Synology site the only power I see there is the psu rating, which is 90W for DS425. So you can expect real power consumption of about 30-40W. Which is typical for just about any NUC or a budget ATX motherboard with a low-tier AMD-something + a bunch of HDDs.
To me, it's a question of time and money efficiency. (Time is money.)
I can buy a NAS, whereby I pay money to enjoy someone else's previous work of figuring it out. I pay for this over and over again as my needs change and/or upgrades happen.
Or
I can build a NAS, whereby I spend time to figure it out myself. The gained knowledge that I retain in my notes and my tiny little pea brain gets to be used over and over again as needs change, and/or upgrades happen. And -- sometimes -- I even get paid to use this knowledge.
(I tend to choose the latter. YMMV.)