What's preventing management from imposing huge amounts of workload and telling you to get it done in 40 hours — or you're fired and they'll find someone else to do it?
I do not mean this at all flippantly. I've worked places (and I'm sure others have as well) where the amount of work assigned is undoable in the amount of time available. People burn out, people deliver poor quality, people quit.
A ROWE is not meant as a mechanism to protect employees from destructive bosses. And, for knowledge workers whose work cannot be mechanically reduced to hours, neither is a 40-hour week.
A ROWE is about removing waste in the work process so that people can focus on the work they're hired to do, not on outdated ritualistic traditions about work that no longer apply.