A Feral Hive is any hive that is not managed by a beekeeper.
Anyway, feral hives might be a hive that swarmed out of a beekeepers hive last year, or a hive that is hundreds of swarms removed from a beekeeper’s hive 200 years ago. You just don’t know. Sometimes you can tell it is completely feral if the bees build brood comb 4.7 mm. My best feral hive is 5.1 mm so I think it is a few removed from a beekeepers hive.
The big deal is in the survivor genes. If the hive has survived in the wild for several years and many generations of swarming, then there is a very good chance it has the genetics to fight small hive beetle, varroa mites, and foulbrood. This is why it is good to bring these ferals into your apiary.
The downside might be a bad temper if they have a bit too much Africanization. Don’t tolerate mean bees, requeen and move on with life. But don’t tolerate having to treat you bees, either. Keep fresh genetics coming in with feral swarms, typically from swarm traps.
Bottom line, bring feral swarms into your apiary will improve your beekeeping by adding survivor genetic stock and improve your apiaries resistance to disease.