I mean, that’s subjective, but “I don’t like the way this system is designed to work” is a very different complaint than “this system doesn’t work as designed.” People decided they thought long adventuring days with many combats sounded boring, refused to run the game that way, and then complained that the game wasn’t balanced. The game was balanced, it was just balanced around a play pattern you thought sounded boring.
The problem with designing combat balance around fewer, harder combats is that it’s much swingier than an attrition model. Variance favors the less likely outcomes, and in D&D, the PCs winning is the most likely outcome. The players are statistically favored to win, but fewer combats means fewer dice rolled, which means results are less likely to hew closely to the statistical expectation. Therefore, the more rounds of combat the system expects, the more confident the system can be in its expectation of the results. If you build a system around the expectation of fewer, harder encounters, that difficulty therefore has to come more from the inherent swinginess of the dice, which means more combats where you either feel like the bug or the windshield, instead of feeling like a well-tuned challenge that you have to use all the tools at your disposal to succeed at.