There is no question that slots can be rigged in favor of the house. Slot machines are typically controlled by computer programs that keep track of how much money players are putting in and taking out, as well as how much time they are spending playing.
This information is then used to adjust the payout percentages in order to ensure that the house always comes out ahead.
Although it is possible to rig slot machines, it is not easy to do and would require a great deal of coordination between the casino staff and the programmers responsible for the machines. In addition, there are often surveillance cameras monitoring the machines, so any suspicious activity would quickly be noticed and investigated.
Overall, it is fair to say that slots are rigged to some degree, but it is not nearly as widespread or systematic as some people might think.