For what its worth I regularly got the same authentication failures with a ReadyNAS NV+ but hardly ever get them with a ReadyNAS Pro. This might be due to the fact the NV+ being a Sparc system has an older and apparently no longer as well supported firmware compared to the Pro which is an Intel system and has the very latest firmware. The other possibility is the NV+ has less memory and a slower CPU.

Anyway, the upshot is that network Time Machine backups work fine on the Pro with very rare instances of authentication failure. Unlike the Drobo it works exactly like an Apple system with no need to do any special tricks.

It should be noted that at least on their newer (Intel) based servers, NetGear's ReadyNAS typically has a much newer release of NetAtalk than most of its competitors, see http://www.netafp.com/status-of-netatalk-and-afp-support-by-nas-vendor-322/