The beach at Denarau certainly isn't postcard material, but I've swum (several times) at the beach in front of the Sofitel. It's not dirty, but it's more akin to swimming in a harbour than at a nice beach. The sand is more mud than sand.
On our next trip we're staying at the Radisson for three nights. I've been there before, for lunch, and it looks fantastic, nice gardens, really nice pool, the food is great. (Also stayed at the Sofitel, which is really nice but with a less developed and more open style of garden. Brilliant food and cocktails, though.)
Denerau is really nice - for what it is. I wouldn't discourage anyone from visiting there. The islands are quite a different experience.
If you don't mind rushing around a bit, and taking on a new trip each day or two (and maybe facing disappointment at the end of those days), do it as planned. But personally, I'd be aiming at maybe three days at Denerau, and the other five in one other place. (In my case, that's Octopus on the Yasawas. It's like living in a postcard.)
I wouldn't want to do day trips to resort islands from a base on Denerau, unless I was reconnoitering for the next holiday. I would do excursions from Denerau to local villages, attractions, or Nadi. Denerau is perfect for that.