There is a discussion of the merits of the Drobo 5N NAS vs the NAS boxes from other NAS vendors such as Synology and QNAP over at the macrumors forum. The discussion kicks off with the question of whether the Drobo 5N is overkill for Time Machine backups.
As you might expect there are a variety of opinions. Some recommend Synology or QNAP because of their history of reliability. Others defend the Drobo because of its extremely easy setup. And, there is even a recommendation for the Drobo 5D which can be attached directly to the Mac.
This post sums up the exchange fairly well:
We use a Drobo 5N for this exact purpose - and it works wonderfully. The 5N is attached to our AVM Router via Gigabit and TimeMachine backups happen wirelessly. It sits in a cabinet and we would´t know it´s there if we hadn´t set it up ourselves firsthand.
That said: Anything from Qnap or Synology offer top hardware. Where they come above Drobo the company is communication, especially problem solving. They offer a different subset of functions, though a Drobo can´t deliver. But a Drobo has its merits, too. And you won´t find a dedicated 5 Bay NAS box with its "HD drop in and forget" handling from anyone in its price range.