A difficulty that I find is that metric as commonly used is more awkward for estimating.

For example, I find that metric distance as commonly used doesn't have a good equivalent to feet for working with medium lengths. By "as commonly used" I mean that, while I could theoretically use decameters, in practice average people seem to only use millimeters, centimeters, meters, and kilometers. That makes it difficult for me to estimate medium lengths in metric. For example, it's awkward to describe someone as 1.83 meters or 183 centimeters tall. It is much easier to describe them as 6 feet 2 inches.

To a somewhat lesser degree, the same seems to be true for volume. In practice people only seem to use milliliters and liters. That leaves me with no good equivalent to cups or gallons.