darbys and h framed darbys are designed for you to hold the full blade of it as flat as poss to the wall and apply a fair bit of pressure so imagine them as a 4 foot trowel- perfect for k- rend or render if you want to close the surface right in
you can use them inside but i prefer heavy duty feather edges, the reason being if you look down the blade of a darby after 6 months it'll probably be all over the place a feather edge may but wont to the extent of a darby due to the heavier duty design,
then when you bear in ming your holding youre darby flat to the wall it'll only be as flat as your darby(doesnt really matter outside), if you hold a feather edge 90 deg to the wall on a final rule you can't go wrong