Many years ago I was an engineer in a metal stamping plant that heated and then stamped metal from sheets of 1/4" to 5/8" thick steel using mechanical and hydraulic presses up to 3,000 tons. It seems to me that Kubota first bent the sheet into the channel shape and then put the curve in the channel in a second step. The only way I know of to prevent the crinkling on the edge would be to form the entire piece in one step in a draw die that forces the extra metal out the edge. Such a die would be very expensive, and an exceptionally large press would be required to handle it.
As for the structural defect, it seems to me that those ripples don't materially weaken the edge that will generally be in compression. And it is entirely possible that the Kubota engineers did not use the lower edge beyond the weld in calculating the necessary strength of the boom. If that is the case, the rippled metal is unattractive, but certainly not a structural defect.
I have three Kubotas and am aware of the shortcomings in each one, from the weak (sometimes nonexistent) power steering in the B-2400 to the cheap plastic radiator screen of the
L2900 to the over sensitive HST control of the
L4330. But I think it is a stretch to call the wrinkles in the lower edges of those booms a structural defect without making a much more in depth analysis of the stresses that are present.
As for John Deere, they apparently bent channel and then welded it together. Since the bottom of the channel (inside of the curve) did not wrinkle, that means the outside of the curve (top of the channel) must have stretched. So the upper surface of the JD boom is thinner than the lower surface. Is that a structural defect? Only if the engineers did not take it into account is calculating the stresses on the boom.