Yes, especially using descendants of the root drive, that is very bad because it will cause a recursive search of all files and folders of the system, which is a HUGE delay.
If you use descendants in a more limited way, like a very specific folder, it does not cause as significant of a performance penalty, though I would still use caution.
The penalty can be reduced by using descendants in an analysis which gives you the ability to control how often the property is evaulated, like once every 12 hrs.
On windows, this is returning an object and by default it is showing you many of the properties of the object, but not the full pathname. The advantage of the object is you can do many different things with it, including returning the full pathname if you wish.