TimeMachine backup over network - are OSX Server or Time Capsule more reliable than TM -> NAS? Time Machine famously has problems backing up to a network drive ...just the other day I got the dreaded message where TM wants to wipe my existing backup and start from scratch again :(

(that's from TM on my laptop going to a 'Time Machine-enabled volume' on my Drobo 5N)  

I have tried Carbon Copy Cloner and Superduper but they both had trouble even completing the initial backup - they don't seem to handle network hiccups well at all and just give up... the part that TM does ok with.

Most advice seems to be to don't backup up to a network drive and use a locally attached disk instead, due to fundamental technical problems with sparsebundle files over network etc.

But I really need to backup to NAS... we have have two laptops at home for a start, I can't plug them both into the backup drive at the same time so there'd be no way for backups to run automatically.

It's a bit odd then that Apple TimeCapsule offers network backups - does it suffer same problem?  I'd buy one if it works reliably, though I don't like that it's only single disk and no RAID.

Then I saw that OSX Server also offers Time Machine backup over the network to clients. Does this work any more reliably than the desktop TM -> NAS drive scenario?  I'd consider getting a Mini and locally attached disk if I could be sure that would work.
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.