Having tested the crap out of it, it's 99% about the tightness of the rack.
If the rack is perfect, using a magic rack or accu-rack, the margin for error is huge. It's drop-dead easy. You can't miss it.
You can hit anywhere between 8 mph - 30 mph. You can break from right next to the rail, the box, even a few inches off center if you cut the head ball a hair. You can put whatever spin you want on the cue ball.
With a perfectly tight rack, the wing ball goes in over and over. Sometimes you completely mis-hit the rack and then the 'wrong' wing ball goes in. It's hard to NOT make the wing ball unless you hit below a certain minimum speed (8mph is my guesstimate).
If you don't have one of these racks, then you're at the mercy of the gaps. If you have watched tucker's videos then you can learn how to read them, but I wouldn't bet your bottom dollar on the wing ball going in. If you overlook some paper thin gap somewhere, then careful aim and speed control won't help.
The more irregular the balls, the older they are, the dirtier they are, the worst it gets.
I'd say the percentages are something like this:
With MBR: 95-99%
With good triangle & newish or regularly-cleaned balls: 80-85%
With old, mismatched balls, or a crappy triangle: 35-40%, maybe 50% if you really read the rack well.
If your wing balls aren't going in, don't invest time working on your speed or break angle. Invest it in getting the rack
perfectly tight.