>>Another problem I see in SI is the definition of the kg. (but this doesn't affect the end-user much). Second and meter have been defined in terms of fundamental constants, but for the kg., a "prototype" is still used for the definition.
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>I thought that the gram was now based on a multiple of the rest mass of the Carbon-12 nucleus. Maybe not.
That is the atomic unit (for indicating the atomic mass of some atom), not the kilogram. Both Oxygen-16 and Carbon-12 have been used for the atomic mass; I forget which is the current one.
AFAIK, alternatives are being explored, but the kg. is still based on a prototype - as was the meter (metre), decades ago. The meter has now been changed to a reproducible standard: a multiple of a certain wavelength.
Something similar happened to the second - it is based on a multiple of a certain vibration at the atomic level.
It seems that the problems in these definitions are practical ones: how to obtain a mass (length, time, etc.) that can be measured with an error of less than, say, 10^(-8).
Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire... (from Gulliver's Travels)
