Amazon PPC accounts fail in a predictable way. A seller launches campaigns grouped by product category, adds more as the catalogue grows, and eighteen months later has 140 campaigns competing against each other on the same keywords with no view of which SKUs actually make money.
The fix is to restructure around profitability tiers rather than categories. Tier one is your profit engine: high margin, high conversion, defensible rank. Tier two is volume that pays for itself. Tier three is everything that exists for catalogue completeness and should never see aggressive spend.
Once campaigns are tiered, negative keyword hygiene, dayparting and inventory aware bid rules do the rest. Across our marketplace accounts this architecture has produced an average 22 point ACoS improvement while unit volume grew.