What is the hardest part of replacing a refill valve?

The hardest part of replacing a refill valve is often accessing the valve itself, especially if it's located in a tight or hard-to-reach space. Additionally, ensuring that all water supply lines are properly disconnected without causing leaks can be tricky. Aligning the new valve correctly and making sure it functions properly without any leaks afterward also requires careful attention. Properly sealing and testing the installation can be time-consuming and challenging for some individuals.