Best-of-breed is a buying approach that picks the strongest specialist tool for each function in the stack, one leading product per job, instead of a single suite covering many functions adequately. It optimizes each component and assumes nothing about how the components connect.
Best-of-breed builds a stack by choosing the leading specialist product for every function: the strongest email platform, the strongest analytics tool, the strongest customer data platform, each selected on its own merits. The alternative is a suite, one vendor covering many functions at merely adequate depth. Best-of-breed trades suite convenience for specialist capability.
The assumption it skips
Choosing the best tool for each job is a decision about the tools. It says nothing about how those tools run as a system. Best-of-breed assumes an integration and governance layer will be there underneath, the part that makes independently chosen components share data and workflow logic. That layer does not arrive with the products. Someone has to design it, build it, and pay for it.
Where the integration debt lands
Buy five excellent point solutions expecting them to cooperate, and you get five systems that do not share data, do not share workflow logic, and need custom code at every connection. The custom glue accumulates into a stack that technically works and constantly breaks, held together by integrations no vendor supports. The stack fractures at exactly the seams the buying approach never accounted for.