I did a breakdown some years back. I get ZERO serious reform amendments in 223 years! As for it being difficult to amend... sure, it SHOULD be difficult. But it's an insane formula where now states with 4% of the US population can block any reform, yet states with 40% can ratify any amendment. The Constitution simply has no internal protections against demographic trends any more than it has against the corruption of elites (financially or ideologically) who were given a veto over the People at every turn.
Here's a breakdown of amendments by category... feel free to break them down in other ways:
INDIVIDUAL & STATES RIGHTS: 1-10 plus 13th, 14th
FINE TUNING THE CONSTITUTION: 11th, 12th, 16th, 20th, 22ed, 25th, 27th
PROHIBITION & REPEAL: 18th, 21st
EXPANDING VOTING RIGHTS: 15th, 19th, 24th, 26th
MAKING THE CONSTITUTION MORE DEMOCRATIC: 17th, 23ed
The first ten amendments, aka The Bill Of Rights, were demanded by the states as the price of ratification. So that leaves 17 amendments over 223 years or one amendment every 13 years.
If we take away the 7 that I've put into the "FINE TUNING" category that leaves 10 amendments over 223 years or one, on average, every 22.3 years. These amendments cover things like presidential terms etc.
Take away Prohibition and its repeal... that leaves 8 amendments over 223 years giving us one amendment averaging about every 28 years.
That leaves 6 amendments that in some way make the Constitution more democratic... that gives us one amendment every 36 years. These amendments fall into two categories.
The first category is expanding the vote to groups who arguably should NEVER have been denied it: slaves (15th), women (19th), those who can't afford a poll tax (24th) and 18 year olds (26th).
The second category deals with some aspect of the antidemocratic structure of the Constitution itself. Here we have but TWO amendments... giving us ONE reform amendment, on average, every 111 years. Those reforms were allowing direct vote for the Senate... and giving EC votes to those in Washington DC. Given how antidemocratic the Constitution is, those reforms are minor tweaks.
The sad reality is NONE of those 27 amendments to date have reformed ANY of the core antidemocratic features of the Constitution all of which are connected with the antidemocratic concept of state suffrage... the EC, the Senate, the exclusive powers of the Senate to ratify judicial nominees or treaties, the amendment process, etc.
That's ZERO serious reform amendments in 223 years!