I can't see a clear reason why a dry top with attached spray deck would be any more hazardous than wearing a conventional spray skirt with a dry top. Granted, the spray deck needs to be released to exit, but that is true for both.
I do know of people who came out of a kayak fitted with an all neoprene spray skirt, slipping right out of the tunnel leaving the skirt still on the cockpit coaming, and I did it once. That would not be possible wearing a dry top with attached spray deck, but most dry tops have the spray skirt tunnel covered by a gusset on the dry top, so I doubt you could slide out of the skirt anyway.
Spray skirts do present a slight risk of entanglement during a swim and I know about one kayaker who drowned when an underwater limb happened to go right through the grab loop of her skirt during a swim. I suppose in a freakish event like that, a separate spray skirt might allow a slightly higher chance of disentangling yourself, but I think you would have a better chance of releasing yourself by cutting through the grab loop with a knife.
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