I'm not discounting
fcorbelli statement "...13.0 is, for my very very low trust level, broken on the ZFS implementation. …"; but would like to point out my experience/opinion:
I've had a couple of machines using ZFS for quite a while. One has a mirror pair (used for home directories) that actually started out as a singleton way back in FreeBSD-9 and has been migrated system upgrades and new hardware and making it mirrored. Currently running FreeBSD-13.0-RELEASE on it and have not seen any issues. It's relatively bog-standard ZFS use, pretty much default properties, worst thing is "NFS exporting". The boot on that system is a mirrored pair that was from probably a fresh 12.x install, then freebsd-updated to 13.0-RELEASE.
Another system was 12.x, freebsd-updated to 13.0-RELEASE, single internal M2 device.
Following is my opinion:
I have not seen any issue in my normal use cases that would support the "...13.0 is broken on the ZFS implementation".
If you go back through the mailing lists, there was a lot (really a lot) of discussion about switching ZFS implementations and lots of call for testers and lots of back and forth getting things fixed. Is it possible that some corner cases were missed? Absolutely possible, but overall a lot of effort was put into getting it right.
Areas of concern are always:
Compatibility. Different ZFS implementations always had the potential for adding features/flags that conflict. If you don't use them, it's not an issue. Think before doing "zpool upgrade"
Bootloader. Always an area of concern if you are using Boot Environments. What if the gptzfsboot or efi bootloader code is wrong and can't recognize my pools? Well, that's happened in the past and yes, it is frustrating, but you often aren't alone. Searching the forum here has a few threads about it and how to fix it. Typically, answers are tied into "be careful if you zpool upgrade things that affect boot environments".
In the end I lament "dropping a FreeBSD implementation for a Linux one", but in this case the switch was "small interest group, pushing patches upstream, seeing them not get incorporated" for "upstream rebased to a larger, more vibrant and active group of users, so FreeBSD should rebase to take advantage".
Again, I am not discounting or diminishing anyone's personal experience, but I too would like to know what is really broken on the current ZFS implementation (vs what acts differently).