Barbarian is not a particularly swingy card. Swindler convert your early $5 into a Duchy, Barbarian just downgrades it to a $4.
I also fail to see why you equalized junking with downgrading a Province into a Duchy. Sure, once you hit that Duchy again you could claim that Province got swindled into an Estate (goes faster by the way in a Swindler/Peddler Kingdom) and thus we have some form of junking but that is really a stretch.
Rambling on about how supposedly overpowered a card is while ignoring what it actually thus is, well, just rambling.
I think there is a fair argument to say that some people find cards like these "unfun," regardless if they are actually overpowered or not, which like stated earlier in the thread, is more what is happening here.
Also, I have played one game where it was a single buy board with Barbarian along with good trashing, draw, and villages--I personally thought it was interesting but could see people say it was "unfun"; winning involved trying to play more Barbarians than the opponent and hoping to trash their victory cards and good action cards that they could only pick up one at a time. It was almost like knights except the knights didn't take each other out, prolonging the game where multiple attacks with random outcomes are being played each turn--like I said, not usually overpowered but situations like that come up where it can be "unfun" for some people.