Boston City Hall isn't just awful because of the deranged architecture (which admittedly is a consistently abusive architectural style), it's awful because it's also a place that nobody wants to be because of the poor design of the plaza (necessitated by the bad design of the building). So it fails not just in terms of design, but in terms of harmony with the city and the people.

I will definitely give the building credit. It's an exemplary expression of brutalist architecture and it'll be in architecture textbooks forever. If you had to design the peak brutalist building that included how it affected the built environment, this would be it.

The style is objectively bad in the same way that if you say that Starry Night is a bad painting or that David is an example of bad craftsmanship you're be wrong. It's not a matter of opinion. A good way to separate your opinion (beauty is in the eye of the beholder stuff) is like this. I like Blink 182. It's not great music but I like it. The Beatles or maybe the Rolling Stones could be considered objectively good music. I don't particularly care for either, but I don't have to let what I think get in the way of what's true.

You can do the same thing with Boston City Hall. You can compare Boston City Hall to something like the Notre Dame Cathedral. There's a reason why one is considered a global treasure, and the other is probably the most hated building in America. We can converge on truthy understandings of things we don't have good objective measurements for (yet). You know it when you see it.