Are there any instances of features being utilized that the old version of the code is unable to cope with well enough to be able to boot the system?

Native ZFS encryption is the major one I'm aware of

Refuse, or legally can't? Oracle doesn't own the copyrights on commits made after illumos forked from the corpse of opensolaris.

A little of both. Everyone know about Linus' refusal to touch CDDL code, but grub isn't the kernel.

There have been several attempts to add features to the grub zfs code over the years, but there are several maintainers of grub who happen to be employees of Oracle, and typically the attempts go nowhere.

I personally can't recommend using grub anymore. The whole "just make 2 pools" solution is unacceptable, and until Oracle stops gatekeeping, their code becomes more obsolete in my eyes.

Yeah update your zfs file system to gain new features and bam, you can't boot no more.