Most ERP failures are not technology failures. They are scope failures — a decision to customise something that should have been configured, or to configure around a process that should simply have been changed.
Configure by default
If a standard Odoo module does 80% of what a department needs, the remaining 20% is usually a habit rather than a requirement. Changing the habit costs a training session. Customising the module costs you an upgrade path, forever.
Customise when the process is the business
There is always some part of an operation that genuinely differentiates the company — a pricing model, a fulfilment rule, a regulatory calculation nobody else has. That is worth building as a proper module with tests, kept cleanly separate from core so upgrades stay routine.
Build separately when it isn't really ERP
Customer-facing portals, mobile apps and real-time integrations tend to fit badly inside an ERP. Build them as their own services against the ERP's API. You keep the deployment cadence you need without holding the finance system hostage to a front-end release.
A rule of thumb
If you cannot explain a customisation to the CFO in one sentence that mentions money, it is probably a configuration decision in disguise.