The A20 comes with only 1 SATA port (and no other fast port, like PCIe or USB3) and it somehow saturates at 50MB/s when writing, so it's a poor choice for a NAS in general (it's usable, but not great). The RK3388 doesn't even have SATA so it's an even worse choice for a NAS.
yehyeh - they're not designed for server-level applications, which is great... because EOMA68 is not designed for server-level applications either :)
FWIW, I'm using an A20 as a small router+NAS+server and am very satisfied with it: the limited SATA speed is never the bottleneck since I connect to it over wifi on one side (where competition with neighbors and in-house obstacles means I rarely get over 2MB/s) and over DSL on the other (and here in Canada, DSL speeds haven't changed much over the last 10 years).
But for a "dedicated" NAS box that can hold up to 6 disks, an A20 would be a rather odd choice.
Stefan