Help me Understand: Themostats

[PanzerSD]

Here's what I know, or think I know, about thermostats.

It stays closed when cold to a certain temperature then opens allowing hot water to flow out to the radiator and pushes the cooled water into the engine, and when the cooler water reaches the thermostat, it closes partially, or completely, depending on the temperatures, to allow the cooler water to warm up to the opening temp, and repeating the process.

In a FAILURE mode, If closed, It will NOt allow the cooler water the enter the engine, and prevent the now, boiling water to leave the engine, resulting in an overheating situation.

Herein lies my uncertainty:
If the failure is open, Then the entire water volume increases in temperature, to overheat, or when in motion or with increased airflow over-cool.
When the thermostat is stuck open, it disrupts the 'pulse' of cooled water and can't overcome the rate at which the engine is producing heat.