Generally, I'd say you are right. You don't have the certainty you're not being cheated.
However, my personal experience is that I substantially netted positive over a year-long online poker career. I don't have any explanation for it, really, but I can't imagine they'd let more than a few players get away with any real amount of money, and I certainly don't think I'm the one that won the lottery of being "allowed" to win to attract other players.
If they do adjust the win rates at an infinitesimal rate to allow their house players to win, then they were doing it both transparently and subtly enough that it was still a profitable venture for me to be online playing a solid game. You could potentially liken it to doing business in a lawless part of the world - some times, you're going to get hijacked, but on the whole it's still making money for you.
Now, for all the guys that *I* took money from, yeah, that was certainly a mistake on their part, but it's not like every player can be a winner, can they?