I remember quite liking it when it came back. Revisiting it, I have a sneaking suspicion that the Barbarian and Cavalier classes were power gamer traps. Like, the Barbarian front loads you with a ton of abilities, but then saddles you with not being able to use magic items and an extremely slow rate of advancement. On the other hand the Cavalier also has a lot of abilities, but its code of conduct practically guarantees an early demise.
I have seen suggestions that the UA barbarian and cavalier might have worked for one-on-one play, in which the DM does not have to worry about whether things are fair for the other players because there aren’t any. In the spirit of 80’s sword and sorcery movies, the solo PC barbarian could pick up some NPC henchmen or sidekicks like in the Schwarzenegger flick, or get your Marc Singer on and go beastmaster with a pet eagle or tiger or war mammoth

. The cavalier could have a squire and a cleric chaplain.
I am getting interested in strongly themed campaigns with restricted player options and specialized focus, and I think that if for some reason you wanted to use UA barbarians and cavaliers as written they might work in a campaign focused heavily on one of those classes. There were several
Dragon articles by (IIRC) Katharine Kerr which focused on real world history as a source for campaign ideas, and I think that could work well with these two classes.
You could have a campaign based on one or more of the “barbarian” cultures described by Classical Greco-Roman authors: Celtic and Germanic warriors fighting on foot, or Scythian, Sarmatian, and Alan cavalry. Lots of other options: Central Asian horse nomads like Turks and Mongols, desert camel nomads based on Bedouin Arabs or Tuareg Berbers, jungle warriors of Mesoamerica (maybe run them through C1
The Hidden Shrine of Tamoachan). Most of the PCs should be barbarians, either all from the same culture or from different ones if they want be a company of foreign misfits hiring themselves out as guards and mercenaries in civilized lands, which is basically what happened in Rome, Byzantium, and Tang China. Maybe bend the magic taboo a bit with a cleric or druid reskinned as a shaman from the same tribe(s) as the warriors.
You could have a party of several cavaliers or cavalier-paladins, each with an NPC squire and maybe a few PC or NPC clerics. You could have a semi-realistic low fantasy setting with feudal obligations, court intrigue, scheming merchant guilds, corrupt clergy, etc. Or go Arthurian high fantasy and have the PCs go on epic quests for holy relics, rescuing fair maidens, slaying dragons and giants. In case of killer rabbits, you can always have the clerics break out the Holy Hand Grenade of Antioch... or just run away, run away!

