There are certain capacitors which fail, but I think they fail under use. In my experienced they 'belly out' sort of like cans of food that are going bad. If the caps are small this might be hard to detect.

I've had board batteries die on me after a few years, both times on Macintosh mortherboards. the computers didn't function without the batteries and once replaced they booted up fine. In that case, the batteries were fixed in small plastic cages which allowed a simple removal/replacement. I don't really know what long term storage would do for them but if the computers boot okay I wouldn't worry about them, in other words, I wouldn't replace them until they failed. The computers are probably not worth much more than the batteries at this stage.

Good luck