It's successful at giving people the option to occasionally put some fresh produce on their plates. It's not enough to keep yourself fed all year round, let alone your family or everyone else who doesn't have an allotment. If an acute food shortage hit right during harvest season I might be able to hold out for a few weeks, but any other time of year would be pretty bad.
> It's not enough to keep yourself fed all year round
In 2019 I produced almost 300 Kg of my own food at home without really trying hard, so maybe not all year round, but is possible to notice a difference. The garden had provided me with at least a piece of fresh fruit each day of 2020 and this point is covered until june-2020 at least.
The problem is trying to obtain your food without space, without feeding your plants, and relying in magical products. This never works as intended.
People needs to understand also that producing food at home is more expensive (but the quality is better).
In Britain and Germany, allotment gardening was promoted as a social measure and by all accounts it was successful.