The box is good but I wonder about your beams...  are they strong enough?

Can't tell from the scale of pic, but it looks like a more substantial hanger bar, so it may be a ceiling fan rated box. A check that screws/nails were used instead of only relying on the tab teeth might be good, and wood blocking can't hurt while it's open. Ask the G.C./electrician, as ceiling boxes rated for fans hold more weight for static light fixtures.