1.It completely dodged any decimalization of time( except fractions of a second). [Though decimalization of time had been and continues to be discussed ad nauseum.] The second is now so embedded into the metric system that we would have to scrap it entirely if we changed to any other measure of time. 2. It completely dodged any change at all to angles including decimalization. It no longer defines an angle unit at all, and considers radians to be a derived unit of metres/metre. And degrees are often used anyway, because having an angular measure in radians is ... well ... irrational! 3. It works terrifically well with decimal arithmetic, and horribly with fractions (except for fractions like 1/2, 1/5, 1/10 - that is, factors of 10). Dividing anything into thirds (or any fraction including a factor of 3) is not exactly possible. 4.It still has weird units (some of which are officially “non-SI” metric units) a. The primary unit of weight is the kilogram. That is, the gram isn’t defined, the kilogram is. The gram is derived as 1/1000 kilogram, which CERTAINLY isn’t the basic metric philosophy to pick consistent base units and then apply prefixes. Here the base unit has a prefix, and the un-prefixed unit is derived. b. Although there are prefixes for 10, 100, 1/10, 1/100 (even for 10,000, etc.), they are mostly not used, in favor of only multiples of 1000. (Centimeters is a notable exception.)
c. When measuring really small things, half the time nanometers are used, and half the time Ångstroms are used, which are “deci-nanometers”.
d. Micrometers are called microns. e. 1 J = 1 W * s. But electrical usage is measured in kW-hr, which are 3.6 MJ. In chemical bond energy, kilocalorie is used (“small calorie”). In food packaging, Calories are used (“large calorie”), where 1 Calorie = 1 kilocalorie. (note big C and little c.) 1 cal ~= 4.2 J; 1 Cal. ~= 4.2 kJ. (approximate, due to the particulars of the various definitions of calorie.) I conclude, a joule is a lousy unit of energy to have picked. f. While many things have been corrected, luminance is still pretty screwed up. First, you have watts, candelas, lumens, lux, each meaning brightness, but in a little different sense. But the primary metric unit is candela. However, candela is just the same “candle” power in use before the metric system – literally the light of one (“standard”) candle. But, like the second, it’s been scientifically respecified: even though candles shine infinitely many different frequencies, a candela is the luminous intensity, in a given direction, of a particular pure green light (which humans are most sensitive to) with a radiant intensity (in that direction) of 683 Watts per steradian. Why not of 1 W, or 1 kW? Because it’s about the same as the previous candle (candlepower, foot-candle). Any other color of light, or blend of colors, has to be adjusted by an assumed “standard human eye” sensitivity curve. (And, historically, the candela was originally specified by the glow of freezing platinum equated to 60 candles. That should make you feel warm.)