There are potential population consequences that can lead to genetic/adaptation consequences from too much of just about any aspect of pond mgt and that includes specifically too much structure/cover. Read the Cutting Edge material from PB mag.

I suggest reading the structure archive (not just the pics) including the links to cover types and how structure works. Keep in mind (think in 3-D) that the biggest structures are the pond bottom then pond edges and the water column depth , then any weeds/plants and after that all the add ons like rock piles , ditches , brush piles , pvc structures , spawning areas , pallets etc. So the 20% add ons is a good general #. It is critical to understand cover/structure as it relates to your location and goals and use it specifically to match. Each item is a tool to use and a random cover it all approach is not a wise tool selection IMO.