As the Sill glacier breaks up, the Cascade-Johannesburg couloir can be extremely difficult in mid- to late-season, once the snow becomes discontinuous. I descended it 7/26/14 and found a variety of problems, from near-vertical snow-steps transitioning from snow to rock, to multiple crevasses/bergschrunds requiring some creativity to pass. I don't believe these problems were visible from below. It's probably best done as an early-season climb these days.
These problems will almost always be recognizable from below, but not from above. Granted, the C-J is a lousy choice in late-season (up or down). In my experience, but your mileage may vary.
I have climbed C-J Couloir in September and we managed just fine (we descended Doug's Direct). I would not have wanted to blindly downclimb C-J at that time because certain problems would have been unforeseen - unlike they were on the ascent. I have also climbed the NE Buttress in mid-October and looked over to the C-J and all problems were visible from below. Just like with any other mountain, the ascent is always the easiest.