It's not just "enthusiast PC builds". You can buy plenty of PCs in that middle category of high-but-not-extreme performance without the certifications, Quadros and vendor markup that a full-on "workstation" model has. And they're perfectly fine choices for many professional use cases.

For that market, the Mac Pro is overpriced (the high-end Dell/HP workstations are too), and Apple doesn't make anything more suited for it. That's the criticism. That the Mac Pro is acceptably priced compared to the Dell/HP workstations doesn't matter if that's not what you need.