For me it was definitely maintenance, especially when using property management. Not only does the property management itself have many hidden fees: technology fees, lease renewal fees, set-up fees, exit fees, etc. But maintenance can be just as bad. Any time someone has to go to the property itself it's always $150 or more. One of my properties has been having almost monthly maintenance requests. I try to stay on top of things but it is always something new/unexpected. Sometimes it feels like death by a thousand paper cuts.
For furnished rentals, I’d price the whole replacement job, not just the item. A damaged sofa can mean delivery, assembly, hauling the old one away and getting someone to meet the truck. Keeping the model, dimensions and access notes on file makes that job easier to quote before you’re trying to squeeze it between stays.