The Bec Hellouin model includes a forest garden, and was started on less than ideal land, and would probably hold up very well productivity wise. I don't have my copy to hand at the moment but the research being conducted on the farm suggests a high level of productivity, using permaculture design and techniques along with Parisian market gardening techniques. Exclusively producing food in a forest garden probably isn't the most productive method for producing food but it brings many other benefits, and I doubt anyone says we should exclusively use forest gardens in temperate regions anyway.
Growing in urban spaces I think it's also important to consider things like the ability of the plants we grow to absorb carbon from the air and clean pollutants out of air too. Heck even here in rural devon they were talking on the radio the other day about high levels of pollution in our towns (because everyone has to drive everywhere because our public transport is rubbish and few people get to live near where they live). But I think that's coming back to the fact that a forest garden isn't just about providing food. It's providing food, fuel, basketry materials, medicines, breathing space, pollution reduction, carbon reduction, immune system support, and probably a bunch of other things I haven't thought about depending on how you design your space. I'm just now thinking about how you could stick a glamping pod or similar into a forest garden and suddenly you have a yield you couldn't get from a potato field.