Most who took furnished quickly sell the furnishings and replace them out of their own pocket. You're much better off taking the allowance.
Unfurnished also mean unequipped - you'll have a kitchen sink and that's all. You will need to buy washing machines, dryer, oven, microwave, dishwasher and everything else yourself. I'd recommend Carrefour. You will also need to arrange and pay for the first gas bottle installation, if you get a gas oven, but the company pay for all subsequent gas bottle replacements.
It is recommended to get a water cooler and water bottle contract or a water filtration system. The water is safe enough, but it does have a lot of very fine sand (shows up when the filters are replaced if you get a system) and there is still a bit of chlorine in the water from the desalination process. That causes accelerated hair loss and skin issues from the showers, but ingestion has effects in the long term. If you prefer the water bottles to a filter system (5000AED), then you can also get special shower heads or filters in the Mirdiff mall to remove the chlorine.