I have seen some gamblers complaining about some sites they are using and they keep making losing with knowing the cause. This could be real that is why we need to make sure that we are using a reputable casinos that are well to do.
Aside the provably fair and a casino being reputable it's not hidden that casinos are taking profits from gamblers. Every player needs to choose wisely the machine that has the payback percentage that'll suit their bankroll, because that's where the con starts. On the long run players still lose, so it's not a surprise thing if the provably fair feature is been manipulated by some casinos. Come to think of it, any casino that boldly writes on their terms and conditions “No Provably fair" would gain lesser customers. So, I think some of the casinos add provably fair on site without having such feature. Between not everyone can check or confirm these features. It's good to use reputable casinos like you said, but it's better to know these techniques and study how slot machines work. That will enable easy swapping of casinos whenever they begin to get shady.