I did the quest for gems twice so I could buy 12 more loot boxes and see if anything changed (ie: stuff dropped less often, more often) in this new build on PTR in this small sample size similar to the size I had before. Seems pretty similar.
I didn't count rare or epic sprays/voice lines/portraits/banners in this. Just skins, mounts, heroes, announcer packs.
12 boxes, two 250g rerolls on two separate all common boxes:
1 legendary item (mount)
5 epic items (4 skins, 1 mount)
8 rare items (1 hero, 6 skins, 1 mount)
Shit still looks mighty generous. I keep waiting for the other shoe to drop and it be more crappy a-la Overwatch but that's a lot of skins and mounts in what basically amounted to 14 boxes. I had similar results before in the 20-30 I bought/earned previously. One interesting thing is that sprays, even though they aren't classified as particularly rare, seem to drop the least often. Almost every box had at least 1 voice line and emoji pack.