It's fairly reliable. Due to the random nature of poker, it's very easy to delude yourself into thinking that you're a better player than you are. This is true for hobbyists, gamblers, and professionals alike. Professional players may move up in stakes or even stay at the same stakes and think that they're stronger than the competition and write off their bad performance as a run of bad luck. Runs of bad luck (and good) can significantly skew your results even over a hundred thousand hands.

Because of this, players of all skill levels will play a long time in games where they are not favored. This is part of why it's so important to keep studying both your play and that of your opponents. If you rest on your laurels, those that do study will surpass you and you'll find yourself playing at a disadvantage without realizing it.

Online you're really able to make a good amount off even a small edge, as measured in big blinds per 100 hands, because hands are dealt so quickly and you can play more than one table at a time. For most of my career I was playing ~1000 hands/hour. In reality due to the rake, the cut the site takes from each pot, you need to have a substantial edge to make money, but this isn't as hard to achieve as you might think.