Weight concerns? These DVRs weigh a TON; about the same as a 4U server stacked with hard drives. It's a bit of a snug fit in the back, but they've been mounted and running for years.
That being said, you most definitely can mount servers in a wallmount cab, depending on their form factor. Specifically, you're pretty much limited to mid tower servers that can be converted to rackmount. You most definitely cannot mount most servers advertised as 'rackmount' due their tendency to have a lot of depth. (Unless you're talking about SuperMicro or something.) An HP DL180, for example, will need around 6-10 inches more depth than any wall-mounted cab can provide.
So there you go.