Each individule complete case suggests the structure of the target system. Can't be sure of exact match with prior design problems. Case sources limited, hence less perfectly matching cases used. Case fragments are used and they need to be modified and reassembled. Varying design requirements may require variations in the previous systems.
Available cases are insufficient.
Optimal retrieval would ensure exact match. Context-dependent subcases make that hard to do, as the retrieval process would be required to reason about key aspects of location where subcase to be retrieved will be inserted. Local conflicts occurred as value dependency violations can be fixed by rearranging the order of subcases in reassembly. Gaps are filled by non-CBR synthesizor Generally no much case competition, and indexing features are distinctive enough. With a reasonablly good case library, CBR is more suitable to solve a problem with the given size, while the synthesizor works on subproblems of much smaller sizes not covered by the case library.