Any benefit to using an SSD?
Low power consumption. Dead quiet. For video recording: That's it. Pretty much any modern 5400 RPM hard drive has enough bandwidth for more than enough simultaneous HD video streams. The limiting factors will more likely be CPU bandwidth, backplane bandwidth and network bandwidth.
SSDs have two big downsides in video recording: 1. Lifespan. You're talking about writing and re-writing big chunks of disk space frequently. 2. Cost. I paid about $260 for 4TB of RAID 1 storage in our NAS. That's about what 1TB of SSD would have cost me.
I would have loved to have used SSDs instead of spinning rust, but there was no way I could justify the cost.