A whole lot has been written about the honesty of land-based slot machines. An excellent article by Steve Bourie is titled, "Are Slot Machines Honest?" and originally appeared within the 1999 American Casino Guide. (It may be found online by searching for the title in Google or Yahoo!.) Physical, land-based slot machines have a lot of "variations" (being polite) than can deceive the player. As always, they may be all illegal and do not happen. But read the article, closely.
On the contrary, online casino slots use an electronic "random number generator" or RNG, operating inside a very large computer program that manages the game graphics, the player accounts of those that join the game, the statistics of every hand played, and much more. The random number generator is hard-coded software instructions that don't change. There isn't any "sense switch" (see the Bourie article) that can be flipped to give players worse odds. Regarding control parameters, each utilization of the RNG is equivalent to the final use, and will be the same as the next use. The numbers will come out randomly, although the pattern of randomness never changes. This really is a true good thing about software-based online casino slots.
Land-based slot machines can also have payout percentages as low as 83%, which is just not true with online casino slots. In
safe online casino slot gaming, each machine has the exact same winning percentage as another machines. You can see the payout percentages for many online gaming sites, and also you will see that it rarely drops below 95% - that's, the house makes a 5% profit, on the regular. In fact, most casinos can document that their payout percentages are in the 97% - 98% range. This means that you can spend per hour, gamble your $100 stake about three times, and also on the regular it will cost you $7.50. But players don't gamble to be average: the true reward will be the well-paying win. And also the high-paying win is balanced, remember, by a number of players who lose and finance that win.

So are online casino slots fair? Yes, they clearly are. The software that operates the games is tested regularly by independent testing laboratories (look for logos at the bottom of the casino website), and no game is individually controllable to be biased. Which is a far cry from the matter with land-based slot machines, appropriately called "one-armed bandits".
Players improve odds, they get absolute consistency across all games - but nobody can guarantee a win every time. That is just the nature of gambling, and that's part of the thrill.