I'd argue that If you do your research before purchasing (which it seems the OP is already doing), you won't spend much/any more time configuring unRAID vs a prefab nas (I'm ignoring spencers' case in this as bad parts are equally likely in hand built and pre-fab (although easier to replace in hand built
)).
On the question of quality, even Eskro's dirt cheap build is way overkill for a NAS only. If OP only ever uses it for a NAS and doesn't take advantage of all the other options, I'd say he's still in a better position with Eskro's build than a pre-fab.
My opinion is based on an aggregate of all factors, in general you're going to be better off building to suit. IF as Balinus stated you need something you can pull out of the box and plug in, pre-fab is a viable option. With that being said, assuming that your amount of free time to deal with this is stays constant through the life of the device, consider the following in your decision:
1. If you're like me, you'll out grow your storage in 8-12months.
Pre-fab: replace the whole system
unRaid: buy any hard drive you can get your hands on and stick it in the array
2. Harddrive failure
Pre-fab: all data lost, unless you kept a backup (in which case you need another NAS)
unRaid: Parity drive recovers the lost data over night
(The same pro/cons apply here as #1, some pre-fab NAS' need all the drives to be the same, unRAID could care less)
3. Other hardware failure
Pre-fab: Buy a whole new system or send it in for repair (where you've lost your data and have nothing for a couple of weeks until repair is complete)
unRaid: Salvage from another machine, pick up a part at Best Buy, order from NewEgg
I did a lot of research for a small business client on storage solutions. In the end, I couldn't recommend any of the pre-fab solutions over unRAID. That was before this new solution (freeRAID?) came about which looks promising in that it can run overtop of other OS's