Regarding the ratio of nitrate : phosphate, I've seen it said that LRS – which is pretty much the quintessential frozen food at this point – happens to maintain the Redfield Ratio in terms to how much of each is added upon feeding. This was discussed in detail on one of the BRS Lives with Ryan and Randy when discussing the Redfield Ratio and whether we should care about it.
Other foods will not have as favorable a ratio. It just depends. But feeding an imbalanced food seems like it could be a primary cause of imbalanced nutrients in the tank. While I alternate what I feed, I use frozen almost exclusively; with LRS as the foundation. I've never seen an imbalance in nutrients.
I personally believe that frozen represents a higher quality food source. Yesterday while I was feeding my tank, I thought to myself – what would I rather eat? Fresh seafood that's chopped and frozen, or seafood that's been dehydrated, powderized, mixed with fillers and binders, and formed into a pellet. When put like that, I think the answer is clear.