Why switching to alert through selenium is not stable?

As per your code block there are a couple of issues which you need to address as follows :

• Switching to an Alert : The method switch_to_alert() is Deprecated and you should be using switch_to.alert instead. The API Docs clearly mentions the following :

   def switch_to_alert(self):
       """ Deprecated use driver.switch_to.alert
       """
       warnings.warn("use driver.switch_to.alert instead", DeprecationWarning)
       return self._switch_to.alert
  

• Wait for the Alert to be present : You should always induce WebDriverWait for the Alert to be present before invoking accept() or dismiss() as follows :

  WebDriverWait(driver, 5).until(EC.alert_is_present).dismiss()