The problem here is that even when building is "functional", the needs of the functions change. If the architect does not allow it, or if the building is not well adaptable, the "functionality" becomes "non-functionality".
In my case it was that the architect (who worked in early 1960's) could foresee classrooms with tables, but could not foresee that the classroom tables would have computers connected to a network (as they had in 1980's).
This is a crucial point. I'm told Brutalism was fairly functional when it came into existence; much of it certainly isn't any more.
Building structures with exposed structural materials means there's no space for hidden infrastructure like cabling and power points. Building structures out of concrete in particular is antithetical to modern technology - no one could have seen WiFi coming, but it's a major hurdle in all-concrete structures.
The shift to power outlets and internet connectivity is a major aspect of usability; it's a shame that so much of Brutalism is in colleges and transit spaces where people urgently want those things.
Ie, they tried to make buildings that looked Brutalist, instead of trying to design them to be functional and eschewing (or not paying as much attention to) aesthetics, which (in my uninformed opinion) is what Brutalism, as a movement, was about.