When does a business need a custom CRM instead of an off-the-shelf one?
When the pipeline has rules an off-the-shelf CRM cannot enforce, or when the permission model has to be genuinely fine-grained. Otherwise, buy one — a standard CRM is cheaper and better supported than anything bespoke.
Most companies should use an existing CRM and should not be talked out of it. The case for custom appears when a stage move has preconditions the business cares about, when records must be partitioned between people who work in the same system, or when the CRM has to be the same system that also does the operational work — the fee ledger, the document pack, the deadline — rather than a separate place to retype it.