How is Freenas or other software based systems compared to Drobo/Synology/QNAS?

Hp microserver is the dogs for this stuff. With cash back it's less that £100. It can take 4 drives out of the box and many more if you mod it. It's cheap to run.

Checkout hotukdeals for the current cheapest prices.

 

You don't need a RAID card with FreeNAS since it uses ZFS. ZFS does, however, need a lot of RAM. The rule of thumb is 1GB of RAM per 1TB of space, more if you want it to be a plex server as well. The FreeNAS forums have a lot of info on what hardware to use but keep in mind that those forums are biased towards enterprise level stuff; there's still plenty of people that use consumer grade stuff though.

FreeNAS uses ZFS and the others don't. That's enough of a reason for me but not everyone needs that capability. I suggest reading up on ZFS if you want to know more. Plus, it runs FreeBSD which you might or might not care about. For me, however, that's important.

Very but how "rock solid" it really is also depends on the hardware you use. I don't use FreeNAS specifically but my server does run FreeBSD/ZFS which is the foundation for FreeNAS.

I'm not sure how easy it is to setup FreeNAS to handle Time Machine backups. I know several people who setup their own FreeBSD servers to handle Time Machine backups but it wasn't exactly a trivial process. FreeNAS might have simplified this process. One of these days, I'll get around to setting this up on my server.

ZFS, which again FreeNAS uses, doesn't exactly work that way. You cannot add/remove disks from a ZFS raidz. You basically have 2 choices to expand a ZFS pool: swap out every disk in a given raidz/mirror for larger ones or add another raidz/mirror stripe. For example, let's say you have 6 disks in a raidz2 (kind of like raid6) and you need to expand that pool. You can either swap out every disk for larger ones, one at a time and waiting for the pool to rebuild after every swap, or add another 6-disk raidz2 to the pool. You cannot, however, simply add a 7th disk to the raidz2. Another example, let's say you started off with the 2 8TB disks you mentioned in a mirror. You can expand the pool by adding another 2x8TB mirror to the pool and then the data will be striped across both mirrors.

 

Even Drobo and others highly recommend backing up your data when adding drives to a virtual volume, as there is a finite risk of corruption.

Time machine on anything other than apple approved destinations can be tricky less reliable as I've heard a more robust protocol is used when a Mac is backing up to a time capsule or MacOS server. Your call.

I'd think about repurposing one of your Macs or a used/refurbbed mini instead of using free NAS, unless you are very knowledgeable about how to set it up and run. This is one of those time is money considerations, if your time is free and readily available its not an issue. Its just so much easier to to time machine destinations in a MacOS ecosystem.... perhaps it will cost less than your time is worth