Minimising delamination risk on flex PCBs with a copper plane layer

Can one of the layers on a flex PCB be made as a completely solid copper plane without risking delaminiation problems due to low adhesion between the dielectric material and the copper? Some manufacturers recommend crosshatched planes to improve layer adhesion, but this can be challenging for high speed designs due to not having a solid return plane. Do you have any guidance on safe design limits before delamination becomes a concern?