We use both the ERU and the NVIC. They are not mutually exclusive.
The NVIC is the interrupt subsystem that enables you to get interrupts from different sources. E.g. if you want an interrupt generated when the microcontroller receives a UART message, you use the NVIC.
The ERU is a complex event-driven interconnect structure that connects events and signals to peripherals hard real time. E.g. if you want the PWM timer (CCU4) to change behavior when an analog signal goes above a certain value (ACMP), then you use the ERU to make the connection.
