It would be very useful to be able to search for resources by attribute/custom field. Such an option is widely supported by most other resource management tools available and allows better resource selection and assignment.
Currently I see a workaround by creating a user group for each attribute and adding a resource to all those that apply e.g. a resource group called "Scrum Master", another called "PRINCE2 certified" etc, then you can search for resources that are members of these groups to find the skill sets you require for a task. This is a workaround and isn't ideal.