What’s the most underestimated challenge in large-scale MEP projects?

I keep noticing that in big projects — data centers, office towers, even industrial builds — the real roadblocks aren’t always the flashy technical stuff. It’s often the less-visible headaches: vendor coordination, late procurement, unclear design ownership, or testing/commissioning surprises.

For those of you who’ve been in the trenches:

• What’s the one challenge that always gets underestimated?
• Do you think it’s a planning issue, or just part of how these projects unfold?

Curious to hear real experiences — both from contractors and consultants.