SSD arn't perfect, they are filled with billions of transistors that will create electrical noise, whether or not you can hear this is up to you. Although while not in use they are more efficient, but when all those transistors fired up, a ssd generally uses more power then some of the more energy efficient hard drives like a WD. A WD 2.5" drive will typically use 350mA consistently, where SSD's typical use 700mA to 1600mA while in use.
Hard Drives have there own disadvantages aswell, typically this is audible noise, although personally this doesn't bother me as I need to hold my ear up to the drive to.... Well hear it.
As far as reliability, niether is really any better then the other. A hard drives magnets are typically the first to go and a ssd, generally the memory controller dies.
Finally you are left with price, hard drives are still all lot less expensive then SSD's, for some people that maybe a non issue.
Personally if your considering dropping $500 for a 1TB solid state drives, perhaps you should see about just having an electrician run a network cable in the wall, It could be the same amount of money and you keep the benefits of using the NAS.
Something to consider
Cheers
Chris
