Often these were originally unrelated, and were only fixed relative to each other fairly recently. Not only are these units unrelated to each other, but they ignore the underlying theory. This is horrible. This lack of relation to the theory also complicates the formulae used to calculate mechanical quantities. Not only are there un-needed conversion constants, but unwanted physical factors, such as the strength of gravity in the power units, crop up too.
Old units hinder general understanding.
Similar problems, but much more significant, occur as the comparisons get more remote from experience. So, I'm convinced by SI, and have been convinced by metric in general for the last thirty years. Many, though, are influenced by the history argument, and many more by the convenience of old units in simple daily activity. The more one uses measurements, and thinks quantitatively about the world, and talks to people in other walks of life, the more SI is attractive.