I’ve not had it happen, but I imagine it’d be the same as if a sata drive failed. There’s no fault tolerance, as you pointed out. My understanding is that each drive has the same directories, and the pool shows all the files from all drives. If a drive goes offline those files should disappear.
I use snapraid to add fault tolerance. Really basically it takes a snapshot of your files and you can recover back to that snapshot if one drive fails. You might think you’d run into a problem if a drive failed, because it might just think you’d deleted a bunch of files, but I believe the default behavior is that it throws an error and notifies you if you have deleted more than a certain threshold of files. That might not be built into snapraid. It might be part of snapraid runner, which I would recommend you use anyway to make it easier to deal with.
So basically, you’d notice your files disappeared, or a cron job would notice, or snapraid would notice, then you’d go plug the drives back in.
I get the concern, but if you’re that concerned about reliability then you should probably use some commercial product that won’t require much know how or intervention.
I’m loving the flexibility of mergerfs, snapraid, and a diy nas. When I run out of physical space I’ll likely just add a few drives in a USB enclosure, so I definitely wouldn’t try to persuade you not to.