The absence of such controls could indeed compromise the system's ability to scale effectively and maintain security against malicious attempts to disrupt service through flooding.

The concept of difficulty adjustment, commonly associated with blockchain technologies, plays a significant role in maintaining network stability and security. By dynamically adjusting the computational effort required to perform certain actions within the system, it helps in regulating the rate at which operations can occur. This not only mitigates the risk of spam or DDoS attacks but also ensures that the system can adapt to varying levels of participation and computational power available across the network.

Furthermore, the implementation of rate-limiting mechanisms specifically for BFT messages is an essential consideration. BFT systems are designed to achieve consensus within a distributed network, even in the presence of faulty or malicious nodes. Without proper rate limiting, these systems could be overwhelmed by a high volume of messages, potentially leading to delays in consensus or, in worse cases, rendering the system non-functional. Effective rate limiting would involve setting thresholds for message transmission and processing, ensuring that the system can handle sudden surges in activity without compromising its integrity or performance.

In summary, the introduction of difficulty adjustment mechanisms and rate-limiting for BFT messages is fundamental to addressing the scalability and security concerns of distributed systems. These measures are instrumental in guarding against DDoS attacks, ensuring the system remains resilient and capable of functioning under various conditions. Implementing these controls will contribute significantly to the robustness and reliability of the system, facilitating its ability to grow and accommodate increasing demand.