* Using desktop drives, when 24 are mounted in a single chassis, the vibration from those drives will cause a higher rate of failure. Desktop drives were not designed for this purpose and are not made to handle the vibration and heat created by 23 other drives in the same chassis.
* When a single drive in your 24 drive chassis fails, how do you know which one it is? On an enterprise storage array, you have the ability to flash an LED so you know which drive to replace, or you have a direct mapping (shelf 2, bay 16) in the error message. If you built it yourself, how do you know which physical drive is /dev/sdzg?
* Purchasing a large number of drives from the same batch means they are very likely to start failing around the same date. Can you handle a double disk failure without losing data? How about a triple disk failure? Do you have hot spares capability?
* Are you scrubbing your data regularly to make sure those blocks on disk aren't getting silently corrupted over time? If you never read your data or scrub your entire disk regularly, the only time you'll find out if you've lost data is during a RAID rebuild, when you'll get read errors and have complete data loss (unable to read parity from another drive, double disk failure, complete loss of RAID).
* Are you able to use RAID 6 or additional parity disk technology to protect against double disk failures? What if I want 3 parity disks to prevent against a triple disk failure?
All of these considerations are things that will come up when you're designing a storage system. These are things that enterprise storage vendors spend months with engineers designing. Failure modes should be considered. In my experience with hand built PC server storage arrays, the time you are most likely to lose data is when you have a disk failure, and are rebuilding parity from another disk. If you have many disks from the same batch, a rebuild can stress the disks more than normal, and cause a second disk to fail. This is why RAID 6 is becoming more and more popular.
I would caution you to consider that cheap hand built storage usually ends up expensive in the long run, especially when you lose data that you thought was protected.