Done properly they work but require substantial investment of time, sturdy timber and masses of soil, cubic yards of it in fact.
Richard, you are in a very small and VERY lucky group, you have access to piles of well rotted fym and (I think from previous posts) also people with heavy machinery prepared to dump stuff where you can get at it,
Repeatedly I get new plotholders making low level corrals of flimsy timber instead of getting the couch and bindweed dealt with. Then they try filling these corrals with bagged multipurpose compost, a car load adds about an inch to one of their beds and costs more than their annual plot rental.
I'd add another circumstance that they DO work well, for the disabled and less able, provided someone very able does the construction. Wheelchair gardening does work, but the bed needs to be 3' high at least, with hard paths each side and about 4' wide at most.
