Hi mikehall12382.
I am not an expert on this and I hope others will chime in to answer you, as well.
I went to a lecture on kashrus and some of the problems with hecksherim, and they exist to some degree with all agencies who are suppose to continually investigate problems, but would range from less than diligent supervision of a food plants where supervisors call in rather than visually inspect, where there have been names of rabbinical endorsements on the packages of rabbis who were niftar, where endorsements were taken away from a company and they continue to use the label and the agency does not take action after that point.
Also, a new problem, which I personally found, are new hecksherim like the one listed as EK. They claim to be “earth friendly” and “spiritually approved”. In addition, there is dispute about “Triangle K” and regular, plain “K”. I dont know the reasons since much of this is usually by word of mouth, as far as I can tell.
I am confident that OU and Chof K are strong and reliable, but again, for me, this has been by word of mouth.