ZFS works optimally if it knows as much as possible about the underlying physical disk structure. A hardware RAID 1 will present a single volume to the OS which ZFS will then treat as a single disk. You'll therefore miss out on the benefits of error correction / fault detection / checksumming etc.
There's another benefit to using ZFS over HW RAID - if your hardware dies, be it the motherboard, RAID card, CPU etc, you can pull the disks and use them in any other system that is running ZFS, and the entire filesystem is readable. If you use HW RAID, you may have a dependency on that specific RAID card, so if it fails you might need to find a similar model before you can read the disks again. You can mitigate this by having dual RAID cards, but this usually starts to become expensive and complex.
In my experience, for the majority of use-cases that I've used a mirrored disk configuration, the performance difference between ZFS and HW RAID wasn't great. If performance is your #1 priority, HW RAID will offer the best solution. In all other cases, I believe ZFS is the better choice.