The risk that comes from ZFS on Linux not being GPL-compatible
A couple of years ago I wrote about the harm of ZFS not being GPL-compatible, which was that this kept ZFS from being bundled into most Linux distributions. License compatibility is both a legal and a social thing, and the social side is quite clear; most people who matter consider ZFS's CDDL license to be incompatible with the kernel. However, it turns out that there is another issue and another side of this that I didn't realize back at the time. This issue surfaced recently with the 5.0 kernel release candidates, as I first saw in Phoronix's ZFS On Linux Runs Into A Snag With Linux 5.0.
The Linux kernel doesn't allow kernel modules to use just any internal kernel symbols; instead they must be officially exported symbols. Some symbols (often although not entirely old ones) are exported to all kernel modules, regardless of the module's license, while others are exported in a way that marks them as restricted to GPL'd kernel modules. At the same time the kernel does not have a stable API of these exported symbols and previously exported ones can be removed as code is revived. Removed symbols may have no replacement at all or the replacement may be a GPL-only one when the previous symbol was generally available.
Modules that are part of the Linux kernel source are always going to work, so the kernel always exports enough symbols for them (although possibly as GPL-only symbols, since in-tree kernel modules are all GPL'd). Out of kernel modules that do the same sort of thing as in-kernel ones are also always going to work, at least if they're GPL'd; you're always going to be able to have out kernel modules for device drivers in general, for example. But out of kernel modules for less common things are more or less at the mercy of what symbols the kernel exports, especially if they're not GPL'd modules. If you're an out of kernel module with a GPL-compatible license, you might get the kernel developers to export some symbols you needed. If your module has a license that is seen as not GPL-compatible, well, the kernel developers may not be very sympathetic.
This is what has happened with ZFS on Linux as of the 5.0 pre-release, as covered in the Phoronix story and ZoL issue #8259. This specific problem will probably be worked around, but it shows a systemic risk for ZFS on Linux (and for any unusual non-GPL'd module), which is that you are at the mercy of the Linux kernel people to keep working in some vaguely legal way. If the Linux kernel people ever decide to be hostile they can systematically start making your life hard, and they may well make your life hard just as a side effect.
Is it likely that ZFS on Linux will someday be unable to work at all with new kernels, because crucial symbols it needs are not available at all? I think it's unlikely, but it's certainly possible and that makes it a risk for long term usage of ZFS on Linux. If it happened (hopefully far in the future), at work our answer would be to replace our current Linux-based ZFS fileservers with FreeBSD ones. On my own machines, well, I'd have to figure out some way of migrating all of my data around and what I'd put it on, and it would definitely be a pain and make me unhappy.
(It wouldn't be BTRFS, unless things change a lot by that point.)