Most teams do not need a custom CRM, and that is worth saying first on a page about building them. If your sales process fits a standard product, buying one is cheaper than anything we could build, and we will say so before quoting.
The cases where building makes sense share a shape: the pipeline has stages a standard tool cannot express, the customer data has to live alongside operations rather than beside them, or the per-seat licence has outgrown its value at the number of people who only need read access. Those are decisions about economics and process, not about software.
When a build is the right call, the risk is not writing it. It is the handover. A CRM nobody but its author can extend is a liability wearing the costume of an asset, which is why the industry pages below spend as much time on what stays yours as on what gets built.