Other than the increased weight the 690 is a fantastic choice. There is a lot of power in this machine and it does very well on fuel consumption with the EFI. In theory you could do some events without ever changing the oil...if you go by the recommended service intervals of oil every 3000 miles. Most of the 690 owners in the mega thread have not had to adjust there valves in 5000+ miles of riding. In a rally your nightly service might become: air filter, chain lube + adjust, oil every 1500 to 2000 miles depending on conditions. If the valves are set at the beginning of the event you will likely be fine for the whole event. At dakar maybe you do a big service on the rest day
the 450/525/530 bikes are lighter and more nimble in the technical stuff. They can certainely keep pace with the bigger 690 and have proven to be reliable just more maintenance intensive. Valve adjustments are easy and oil changes quick. Most people run an oil cooler to help supplement the stock resevoir capacity. The new 450/530 bikes have seperate oil chambers for engine and transmission.