Risk of shock/electrocution (pity we don't have pictures of my electrician grandfather's blackened arms) and/or falls. Physical problems from extended time in awkward positions. Boiling hot or ice cold attics. Dead animals or worse in walls and crawlspaces. God knows what in the insulation you're elbow-deep in. Every small-business problem you've ever heard of, plus coordination with other contractors who lie to each other every bit as much as they do to their customers. And you can top it off with inspectors who don't actually have the first fucking clue what code is, or think it's both a minimum and a maximum (12 gauge on a 15-amp circuit? NOPE! That's not code, you've gotta use that little 14 gauge crap instead!).