We recently stayed at the Mercure Nadi both before and after our outer island vacation.
Simply put, the Mercure Nadi is adequate as a transit hotel but would not recommend staying there any longer than a night or two. Its accommodation is tired. The rooms while clean, smelled musty on entering and they really need a full revamp (that doesn’t involve just slapping paint on as they have very obviously done in the past). There is no water provided in the rooms so make sure you purchase some to brush your teeth with etc.
Whilst we didn’t dine there at night, the meals we saw being served up looked good, and the breakfast buffet was really good. The bar staff at night could do with further training. The concierge staff were all very good and very helpful.