I'm running unraid for only a week, and I have discovered what bjp said (about air flow at top) by myself also
here is my setup:
The upper disks running hotter that the other by 4-5 C.
Prolly I will put them a bit lower .
This is an old Lian-li V2000 case that I had. There are 6x 5.25" bays in the upper and I can put there some 3in4 trays if I need upgrade.
Here are some other views of my Unraid system (to be on topic )