My only concern would be that the deck be truly and well shuffled.

Cards sticking together, shuffling habits, and other human factors could influence the realistic number of permutations available, and might reduce the total entropy of the seed.

The question (perhaps one that can only be answered by running hundreds of thousands, or millions, of tests with actual humans shuffling actual decks of cards) is: How much entropy is lost to the various biases that might be involved?