They both have a « write hole » problem (or though much less so in RAID 6). Any power failure which causes an incomplete write can cause a complete RAID corruption - meaning all data is lost. Hardware RAID controllers usually have an onboard backup battery so they can store some information to complete operations should there be a sudden power failure. Software RAID does not have this, and you need to provide a UPS with automatic clean shutdown as the battery runs low using nut or some equivalent.
Some people go as far as to say that RAID 5 should never be used.
You also have very long recovery times when you replace a failed drive (days). Any other failure during this time means total data loss (of course RAID 6 gives you a second redundancy). Weekly Resyncs are very slow too (hours to days), and (unless you constrain your max throughput) will bring your system to its knees.
zfs does not suffer from these problems, BTW.
I run software RAID 5 via mdadm and have a UPS. I’ve replaced drives twice with no issues other than a slightly nervous long wait during recovery. I’m too cheap to buy the extra HDD for RAID 6, and may end up regretting it one day.