We had NO problems catching a bus from POR to the parks, but the lines for the buisses to get back to the resort at night were always the longest of any of the other bus queues. We are super patient people, but this really got to be a bit annoying by the end of the week. I would suggest getting to the bus stop before park close, because anytime we stayed for fireworks or anything, the POR bus lines were always a 30-45 minute wait for us to get on a bus. We actually took a taxi once.
I honestly do believe it is all luck and coincidence with these busses. We had a bad experience, but we could have gone a week later or something and it would have been totally different. The resort is lovely though!