As an end user who has both Avigilon (OEM Dell) servers and servers I have spec'd myself, I submit the following experience:
We have ten (10) total servers and about 450 camera streams. Two of the NVRs are the Avigilon OEM models that were available in 2013 - I believe these are the equivalent of the Dell R520 (what is now their "Standard" NVR line). They are decent machines, and in a lower camera density environment they would likely do well. However, I prefer something a bit more robust and versitile.
I began spec'ing my own Dell servers - initially the Dell PE R720xd and now the Dell R730xd. I have been very pleased with these machines. Since I started using this configuration a few years ago, Avigilon has begun selling something very similar in their "Premium" line - I believe they are rebranded R720xd machines (but maybe the R730xd - our Avigilon rep was not sure), and they would likely meet my needs.
I can purchase my own Dells for less than Avigilon will them to me, though. However, I do get discounted pricing from Dell through purchasing agreements. Last time I priced a R730xd with two Xeon 6-core processors, dual 750-watt power supplies, 16GB RAM, flex bay HDDs (RAID1), 2GB PERC, and 60TB RAID5, it was around $3K-$4K less than I could purchase one through my local integrators.
I buy a 5-year NBD onsite warranty, and Dell has never failed to take care of what very few issues I have had (one failed memory module and one failed HDD since 2013). On a broader note, I manage a total of 18 or 19 physical servers - all of them Dell. Since 2008 I have had fewer than ten (and prob more like six or seven) occasions to call Dell for support on one of their servers.