import logging

def validate_slack_event(event_data, max_message_length, valid_event_types=None):
    """
    Validate incoming Slack event to ensure it's a legitimate message.
    
    Args:
        event_data (dict): The incoming event data from Slack
        max_message_length (int): Maximum allowed length for messages
        valid_event_types (list, optional): List of valid event types. Defaults to ['message', 'app_mention', 'app_home_opened', 'event_callback']
    
    Returns:
        bool: True if the event is valid, False otherwise
    """
    # Set default valid_event_types if none provided
    if valid_event_types is None:
        valid_event_types = ['message', 'app_mention', 'app_home_opened', 'event_callback']
    
    # Check if event_data is a dictionary
    if not isinstance(event_data, dict):
        logging.warning(f"Invalid event: Not a dictionary. Received type: {type(event_data)}")
        return False
    
    # Check for 'event' key
    if 'event' not in event_data:
        logging.warning("Invalid event: Missing 'event' key")
        return False
    
    # Check that event is a dictionary
    if not isinstance(event_data['event'], dict):
        logging.warning(f"Invalid event: 'event' is not a dictionary. Received type: {type(event_data['event'])}")
        return False
    
    # Check for event ID
    if 'event_id' not in event_data:
        logging.warning("Invalid event: Missing 'event_id'")
        return False
    
    # Validate event type
    event_type = event_data.get('type')
    if event_type not in valid_event_types:
        logging.warning(f"Invalid event type: {event_type}")
        return False
    
    # Basic message validation
    message = event_data['event']
    
    # Ensure message has required keys
    required_keys = ['channel', 'user', 'text', 'ts']
    for key in required_keys:
        if key not in message:
            logging.warning(f"Invalid message: Missing required key '{key}'")
            return False
    
    # Validate channel and user IDs (basic length and format check)
    if not (isinstance(message['channel'], str) and len(message['channel']) > 0):
        logging.warning("Invalid channel ID")
        return False
    
    if not (isinstance(message['user'], str) and len(message['user']) > 0):
        logging.warning("Invalid user ID")
        return False
    
    # Check message length to prevent extremely large messages
    if len(message.get('text', '')) > max_message_length:
        logging.warning(f"Message exceeds maximum length of {max_message_length} characters")
        return False
    
    # Additional security checks can be added here
    
    return True