Is cycling the toughest sport?

Could you please elaborate on why some people argue that cycling is the toughest sport? Are there specific aspects of cycling that make it more challenging than other endurance sports? Do the physical demands, mental resilience, and competitive nature of cycling contribute to this perception? How do the various disciplines within cycling, such as road racing, mountain biking, and track cycling, compare in terms of difficulty? Finally, is it fair to say that cycling is the toughest sport across the board, or are there factors that may vary depending on the individual athlete and their specific training and competition goals?