So the idea behind the metric system was to have one system that unified measures of distance, volume, mass, temperature, and more based on the natural constants of water. So that any unit can be derived from others based on water. And further to have ranges based on multiples of 10, with set prefixes so that addressing things at different scales need not see inflation of digits. This definition didn't last but you'll still see memes lauding the metric system's superiority because of these relations.
The water based definitions for the metric system didn't last because they were too naive. The idea was that one or two measures plus water would let you derive any third measure, but it turns out that in the real world, it's not that simple and you really need pretty much all the other measures to derive a missing measure. Not to mention that water is seldom as pure as we'd like. So the water definitions were dropped. From official adoption in 1795 this idea didn't make it 4 years before being replaced.
But before the metric system was even out of the gates it already started to be crippled. Maybe it's always struck you as odd that the default mass unit has the kilo prefix. It is odd. The original mass unit was the grav. Unfortunately grav sounds like graff, and graff was a title of minor nobility at the time, when nobility was not in favor. However the milligrav has already taken on a more colloquial name of gram. So they went with that instead, skewing the alignment of the base units and their prefixes.
The system that replaced the water system was prototypes. That is to say they made specific objects and set them aside somewhere to be the definition. Of course they didn't make just one of these, they made many as identical as possible, though a specific one was named to be the actual factual definition. Obviously switching to this system removes the smug superiority from the metric system. It also means that the system is based on items that can get lost or destroyed, which isn't great. It also, as it turns out, was still too naive. These prototypes were replaced at least once, to improve the material science, but that's still too naive, because our technology just wasn't in place. As the duplicates of the prototype when returned and compared to the prototype, were no longer the same. Which really undercuts any faith in the system.
As a next step for the metric system they want to turn them back into natural constants. This has already happened for the meter. They have chosen to define in terms of the speed of light in a vacuum. The consequence of this that I think is the silliest, is that this means you can't measure the speed of light in a vacuum with the metric system. What ever result you get the answer is by definition, not by measurement. This seems like a silly thing to do with such an important constant. Additionally there are just the basic questions about this constant and our knowledge of it. Have we ever has access to a perfect vacuum? Have we had sufficiently fixed endpoints in any sort of vacuum to get a good measure of this? Don't we measure everything else with light, so how can we calculate margins of errors for calculating the speed of light, when those margins depend on the speed of light? And I'm not enough of a physics guy to get deeply into relativity, which fixes it, and makes it so that it the wavelength of light from moving things that change and not the speed, but also has time and space deform. Which when you go from pure mathematics of theoretical physics to the real world still has perhaps some degree of hand waving going on.
For the Kilogram, one idea at least, is to make a prototype in the shape of a sphere out of an atomic molecule, measure it's diameter, calculate the number of atoms in it, and then base the definition of the kilogram based on that. So here material purity rears it's head, and in this case not even ions are allowed. Next you have that they will calculate the number of atoms in a sphere. based on it's diameter, cause nothing says imprecision like including an irrational number. But then you have a similar problem to light, determining the mass of electrons, protons, and neutrons. The mass unit will be defined in terms of the combination of all three of them. So really you can only measure their proportionality. And once you have their proportionality, you can never measure the mass of anything again. All you can do is count it. The only free variables when considering the mass of something will be what it is and how much of it is there. If you have those answers then the mass is by definition, and the proportionality of those 3 particles. Even the mass of other subatomic particles might be in fixed if quarks have fixed masses. This seems monumentally foolish and roundabout to me.
There is probably more silliness in the history of the metric system. However the question that's been avoided is why is the imperial (or any other) measurement system better? Simply stated because they don't try so hard to be perfect, and certainly no one one goes on about why the imperial system is better based on it's vast and constant technical superiority. The smug adherents to the imperial system normally put forth it's cultural superiority, which is an argument that defeats itself.
I think really the thing that irritates many (including myself) is that it speaks to the hubris of people ready to declare themselves in nearly full understanding of the universe. Time and time again such people have been found to have been lacking. And even worse sometimes their well intentioned ideas survive and are used as ammunition against improved understanding, despite having been originally put forth to promote understanding. The math used by theorists is precise and perfect, the real world is not, and only fits approximately within precise models. As we learn those approximations improve. They may never be perfect, but that's good since it pushes us to look deeper. The ethos of the metric system is opposed to that, and I guess that it why it irritates me so much and find humor in the foibles of the supposedly perfect system.