TimeMachine backup over network - are OSX Server or Time Capsule more reliable than TM -> NAS?
When two or more client access the TM service at the same time then a corruption can happen. TimeMachine on a TimeCapsule will create a lock when a backup takes places allowing only one backup being active at any time. Most third party implementations will not mark the TM as unavailable when another backup already is running. 

I have seen sparse bundle corruptions with a Qnap NAS. The advice (on the qnap) forums is to use one client only per NAS. Since I follow this advice (one mac will backup to my old TimeCapsule, the other mac backup to a TimeMachine service on the NAS) I had no more corrupted sparse bundles.

Edit: 
Here is the post in the QNAP forum although the poster does not go into much details.