It was a source of anguish last night as several times I would start to chase a fleeing champ with powerball, only to be immediately knocked out of it simply because there was a minion behind me.

I wish they would fine tune the hit box on that, make it smaller towards Rammus' back so as to not accidentily go off if you're close to a minion when giving chase.

I would agree. There's been plenty of times where I was immediately next to or on top of an enemy champion and I simply wasn't hitting them.