Flights - wierd fare classes that dont have the normal options re changes and cancellation, no recourse through the airline if there are any issues
Customer service - hours on hold, and you have to call them to change anything, the hotel/ airline wants nothing to do with you
Hotels- treated like a 3rd class citizen, no points, cant use your status if you have it, worst room, I have been charged extra "urban fees" at the hotel on these fares. Hotels (business hotels anyway) dont want you to use Expedia and treat you like shit if you do.
I expect they have a niche for infrequent, budget sensitive, travelers as I have seen deals on there, but if you travel regularly, or expect plans to change, stay away.
What I like about google flights is how minimally commercial it is, it really looks just like its reporting fares, not flashing promoted bundles of things in your face.