If governments in Civ 6 are too similar, it's because, by and large, no matter when they occur in the game, they are all based on 20th-21st century governments and their interactions with their population. Historically, government types were much more distinctive, but the distinctions were as much in what they could NOT do, or do well, as in what they could. That makes a more distinctive depiction of them pretty problematic from a game design standpoint: nobody wants to be told that having a God King Theocracy means that your economy is going to suck big time, or having a completely democratic-assembly-type government like Classical Athens means that you can't move the same unit twice without asking permission.
Gamers want Positives and Bonuses, not, as a rule, Restrictions and Limitations.
For me personally, the biggest drawback to the governments is the almost complete Freedom of Choice they give the gamer. I can switch from Monarchy to Republic/Democracy/Communism and then back to Absolute Monarchy (if I so desire) without any major and/or lasting effect on Loyalty, Economy, or Diplomacy.
WTH?
It should be hard, disruptive, and potentially violent (civl war, revolution, etc) to change governments. In fact, some kind of internal strife should be almost a certainty when you change government types, and so it should be a major decision when and to which, if you are playing well, and should require some preparation to accomplish. And a major disruption to your Civ, like losing cities in a war, or suddenly expanding your influence by colonizing a whole new continent, or changing/having it changed for you your religion in most of your cities, should result in, potentially, a change in Governments Not Of Your Choosing.
And the governments you can choose should much more limited by the government you already have, and the Social Policies and how long they've been in place, and even the size of your population. Try creating an Absolute Monarchy and its centralized decision-making in a multi-continent-spanning multi-ethnic (as in, lots of conquered cities and city states) Civ: can't be done, or at least, cannot be maintained for more than a few decades/turns without Unrest, Corruption, Revolt, and all the other historically interesting but game player frustrating Events.
But again, the problem with My Personal Perfect Civilization Game is that too many restrictions and negatives might make a fascinating set of realistic, historically accurate problems in Civ/Government management, but also, and I fear for too many people, make for a massively frustrating and unpleasant game...