Is removing network access to a shared Time Machine drive mid-backup similar to unplugging a physical drive?

I'm considering using FreeNAS to provide Time Machine network shares for Time Machine backups of 5-10 Macbooks. One thing I like about this approach is that users simply have to join the network (e.g. open my Macbook), and the Time Machine backup will commence. This is advantageous as there is no drive that the user must remember to plug in.

The disadvantage is that users will surely simply close their laptops and leave the network, effectively "unplugging" the Time Machine network share.

Given that Time Capsule supports this exact use case (wireless Time Machine backups), I'm inclined to think that suddenly leaving the network mid-backup won't be an issue. It would be nice to have confirmation from the community, though.

Will this cause problems? What should I do to make this use case less error-prone?

The nature of backups is that you have an hourly window of time to start one, so the majority of the time, the majority of the computers are not mounting the network share. The algorithm itself is solid in my experience.

If you are starting the first backup, that can take quite a while to save, but if you corrupt the disk image then, you can nuke it and start over with no loss of data (there weren't any backups to lose).

The OS enables journaling so that the worst that happens in practice is the one interval that was in the middle of being changed gets cancelled and cleaned up. I've had very few OS X server backups have issues with network backups, so as long as the software you run on the NAS is equivalent to OS X implementation you should be just fine with your plan. You can also hedge your bets with periodic backups to hard drives. OS X supports multiple time machine destinations as well as ad-hoc adding and removing them to make more point in time ones that you can archive easier than trolling for changes on the NAS.

Some data points - roughly 100 Mac are backing up to about a dozen Mac Mini over the last 3 years, and the number of trouble tickets to assist with things users can't solve has been 2. One user was able to be guided to mount the backup destination and use Finder to remove the partial backup interval. The other made requested we copy their image to a hard drive from the server so they could get the files they needed. That user used a local HD to do backups in the interim and now backs up to that drive and had us wipe the server side image and is backing up to both destinations with no issues.

We don't manage anything or train users to do anything special. Power off, disconnect from network, they just expect Time Machine to run and we train them to check on it two times: a) before making a large change to a document that losing work would mean losing more than 4 hours of work. b) once a month, run a backup manually from the Menu Bar to ensure it completes.

On the server side, we have visibility into when backup images are not updating for more than 3 weeks by a script, so you can implement a check on your NAS if you desire to recreate that Server.app functionality. For $20 I'm a big fan of using server.app to handle things, but lots of people like DIY too.

After trying WiFi based Time Machine backups on a NAS (MyCloud) and a NAS-like arrangement (USB hard drive plugged into Airport Extreme), I have found no solution where Time Machine 'just works' with disconnected laptops.

Time Machine works perfectly for desktops, Ethernet attached or otherwise. But when used on a laptop, where the user can interrupt Time Machine, I have found hung, stalled, and corrupt Time Machine images frequently. Typically, Time Machine will report a problem with the backup, and start a new one.

The only solution I found was to stop using Time Machine, and instead use a cloud-based backup system that was built assuming this use case was normal. I prefer Crashplan, but there are many others such as Backblaze, etc. My assumption is that Time Machine was built assuming always connected hard drives, and Wifi is not a well supported case. Time Capsule perhaps has some secret sauce, but I have no experience with it.