I guess I'll just chalk that up to pop cultural drift, then. Clearly, the original Ravenloft designers were not drawing from Blade or Castlevania.
But we're all on board with balors needing to be tougher, right? A balor should be able to take a high-level party in open combat?
I think part of the problem is also a misinterpretation of the original design approach. Gary Gygax and Ed Greenwood have both commented that the challenge of monsters, certain spell levels, and levels of NPCs were all set with the expectation that they should be significantly higher than the PCs so they couldn't be easily destroyed. Of course, once they were of a given level, the expectation is that the PCs could gain them as well.
When 2e was released, there were a number of monsters specifically targeted to make them much more challenging, dragons being one of them. When demons and devils were added back, they were more fearsome too. Again, as 2e/2.5e continued, character capabilities caught up and eventually surpassed these. And that continued throughout every edition.
Combined with the fact that many players are fans of other genres, like superheroes, or mediums, like video games, etc., the game took a decided shift away from a more "realistic" approach to a heroic (sometimes superheroic) direction. Ironically, while 2e was creeping in that direction, BECMI was going full steam toward the Immortal set where it was expected that characters could one day become gods. They were often disdained by the AD&D crowd. But I think 3e (combined with the constant power creep of both 2e and the Forgotten Realms) moved toward that direction anyway.
As for the Balor? They were pretty fearsome in 1e, but their biggest defense was the ability to gate in additional demons. They could gate in a glabrezu (80%) or a nalfeshnee (20%). There was a 70% chance in a given round that they would do so, and this was in addition to their attacks. There was no limit to how many times they could attempt to gate in more help. If they didn't gate in more demons, their most likely magical attack was to use a Symbol spell (fear, discord, sleep or stunning). Again, at will.
Otherwise they were 8+8 Hit Dice (8th level monster with +1 Con modifier), with an attack that did 1d12+1 damage, or their whip that did 2d6 to 4d6 damage because they made their ring of fire and dragged you into it with the whip, the damage depending on which of the 6 Balor you met. So no, not that tough. It was all about the minions. Oh, they did have psionics too, if you used that.
AC was -2, or plate with a +4 shield. Once again, I think ACs have been lowered too much for my taste. Also, magical weapons only.
Again, things changed in later editions, as did expectations. Answering your question also requires a definition of "high level party."
For my tastes, all fiends are underpowered, but more importantly, they aren't fiendish enough. Devils in my campaign tend to operate by deceit rather than open combat. Because of the way I handle fiendish access to the material plane, their initial goal is always to find a loophole or method to remain in the material plane. Their goal is always the same - corrupt as many souls as possible to send to the Hells after death. That grants them power in the Hells. Devils killed on the material plane lose standing in the Hells. So they avoid open combat if possible.
Demons, on the other hand, are a source of sheer destruction. So they are looking to cause the most pain and suffering as possible in the short amount of time they are present, or attempt to possess something or someone. They thrive off of the suffering itself, and gain power in the Abyss from it. Mere destruction or killing isn't sufficient - it must be as horrendous and horrifying as possible. Unless somebody voluntarily gives them some means of staying on the material plane, their visit it (thankfully) short. It can be extended indefinitely by possessing something or someone, but they have little patience for extended waiting around. Possession is usually among their most effective approaches, for the average person, seeing a loved one so afflicted feeds a demon constantly, and it's often a means of driving others to horrible acts of evil which will further feed its hunger for chaos and destruction.
The main issue they both have is that getting to the material plane is very difficult. So they each want to maximize their stay. So if a Balor can immolate a group of people upon its death, knowing that a death by fire is terrifying, painful and horrifying, it will benefit upon its return to the abyss with a growth in power if that's all it can manage to do.
Meeting a Balor in the Abyss on the other hand...heh, heh. That's an entirely different story. The MM stats don't begin to cover that.
As you can see, my concerns with monster design in D&D has far less to do with the stats, and much more to do with the concept.