REX’s winds aloft are not great. They are somewhat in the direction of the model, but do not represent the correct speed. The upper temperatures are way off. The temperature follows a linear gradient and if you get above FL400, temps are -100C. That causes problems with turbofan thrust settings.
Unreal Weather will let you set updates to any time period and REX will only go as low as 5 minutes. This can be a problem when you are flying a fast jet through a front and weather is changing quickly.