So basically if you have an object that will have multiple instances, and you aren't destroying those instances all the time, you might want to use uid for picking.
But the best way to use that method is to do it at runtime, and store the objects uid on creation.
Otherwise, if you want multiple instances, that may be destroyed at random times, use pick "nth", or sprite().
Not that pick nth cant be used for objects that wont be destroyed.
And keep in mind its much easier to pick chains of sprites using this method as well.