Statistical analysis of slots
The physical number of symbols per reel is meaningless, just as it is in land-based slots. The software maps many 'virtual' stops onto the limited number of physical symbols on each reel. That rules out any visual counting approaches. However, the statistical approach is not as daunting as it seems, once we can establish one key point.
The reel stops must be independent of each other. In other words, if you see a jackpot symbol on reel one 1/20th of the time, and on reel two 1/30th of the time, and on reel three 1/40th of the time, then you should expect to line up the three winning symbols 1/(20*30*40) of the time.
That assumption doesn't have to be true in the world of online gambling. Instead, online casinos could choose to pick a payout amount, then randomly choose a reel display that corresponds to it. That's certainly still fair in the world of slots, but taken to an extreme it could make a machine 'feel' like it should hit more.
Imagine a machine where the jackpot symbols show up roughly half the time on any given reel, but where lining up all three of them only happens once in a million spins on average. Still fair, but definitely misleading.
My current research indicates that MicroGaming slots don't play these kinds of tricks, but instead mimic what we would normally expect at land-based casinos. This means that we can begin to draw useful conclusions from relatively small amounts of data gathered. For example, I recorded exact reel positions for 400 spins at Fruit Fiesta. Using the symbol frequency, I calculated the breakeven point at a whopping $229,000. At the starting jackpot of $800, I estimate the return on this game at 88.5%. Now, 400 spins wasn't enough to be very confident, because I only saw the Fruit Fiesta on reel 3 in the winning position once in my 400 spins, and therefore estimated its frequency as 1/400. But, hopefully you see that we don't need millions of data points to make a decent approximation.
I expect the breakeven points on some games to actually be reached occasionally, unlike the Fruit Fiesta game, which is prohibitively high. However, as you mention, the variance will be huge on these games, even if they become +EV.