Are you backing up to the Synology over WiFi? If so, that will be a small problem.. where "problem" = time and network speed. By going WiFi, you're limited to the highest negotiable speed your router can handle for WiFi. If that is 802.11g, for example, you're limiting the transfer speed to no more than 54Mb/s. That's slow, depending on how much data you're backing up, especially compared to if you're going over something ethernet. Even USB will be faster than that.
That's why despite my having a Synology as well, I'm backing up to an external USB HDD. When I order a new Mac, I'll be going Gigabit ethernet to my router, then can back up to my NAS.
BL.