In my experience (in ~30 datacenters worldwide, and reading about more), the actual -48v Direct Current plant is usually ROCK SOLID, in comparison to the AC plant. It's almost always overprovisioned and underutilized, at least in older facilities, or those with telcos onsite (who, unlike crappy hosting customers, actually understand power).
My pro tip for datacenter reliability is to try to get as much of your core gear on the DC plant as possible -- core routers, and maybe even some of your infrastructure servers like RRs, monitoring, OOB management, etc. Ideally split stuff between DC and AC such that if either goes down, you're still sort of ok, or at least can recover quickly. DC and AC is even better than dual AC buses, since what starts out as dual AC can easily end up with a single point of failure later (like when they start running out of pdu space, power, or whatever), and dual AC is also more likely to have a closer upstream connection.
DC stuff is WAY simpler to make reliable and redundant, just uses larger amounts of copper and other materials.