I am seeing some of these reports, but even where they are found it seems few and far between.
In article about workers finding a body by chance you also had the police spending C$500,000 to see if it would even be feasible to search landfill for bodies of women suspected to be there. People being found is the exception, and maybe they're particularly hot on it in Canada - but it's not commonly successful.
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There is no fool-proof way of doing any of this. Every option presents risks - if you opt to melt people in acid then there is a lot of preparation, which inevitably leaves a financial trace, and time. Nevermind disposing of the chemicals. Sure, there may not be a body to find in the end, but they don't need to find one.
Landfill is always a good option especially if their disappearance wouldn't be noticed initially (like with Mama JF). It requires a relatively low amount of effort, is incredibly hard to detect once the corpse has been deposited and overtime the chances of the body being discovered becomes basically nil.
Of course, the most effective way to not get caught killing people is to not kill people. What always fascinated me about criminals is their willingness to throw their own lives away over what amounts to nothing.