On the flip side if something goes wrong you get to discover what $RANDOM_VENDOR in $RANDOM_COUNTRY thinks is a reasonable policy for handling it, which is not usually a big risk for domestic USA travel, but for international travel can be a true wrench in the spokes.

Maybe but if $RANDOM_VENDOR isn't willing/able to help, I don't really expect Expedia to. That said, I do use an agent to arrange self-guided walking trips and things like that and they seem to be a useful resource rather than planning and booking the whole thing myself, in part, because a lot of local knowledge can be involved. But that's different from booking 5 nights in some European city.

I had a couple of times when car rentals and hotels decided to add surprise charges when I arrived. As those was booked through Expedia, the latter refunded me those immediately (and I suspect went and got them back from the vendor later)