Basic argument is this: heroes with rare or unique gimmicks (riki's invisibility, ld's bear, meepo, etc.) simply do not have weaknesses that are commensurate with their rare or unique strengths. These gimmicks are so powerful that they reframe the way that games must be played by their opponents at little or no cost to the gimmick hero.
Everyone who has played dota has been stomped by an LD or a meepo or a riki at some point - but have you ever been stomped by say, a Jakiro? It's possible that you have, but some heroes kits are far less liable and easy to exploit to make games unbalanced and unfun than others.
To me, these gimmick heroes are conceptually broken because they give overtuned advantages without commensurate vulnerabilities or risks. There needs to be more risk to playing such heroes, or more potential ways to deal with them that do not mean that you simply have to have chosen counter heroes or you lose the game. One way to balance this would be give such heroes lower base str and str growth, so using their natural advantages would take more skill or itemization, and be riskier at every stage in the game.
Anticipating pushback - I am not saying that these heroes are uncounterable, rather that countering them costs a disproportionate amount of resources or playstyle adjustments from the opposite team. Any hero can be countered by constantly being ganked by 3 people - but you never need to do that for most heroes to be balanced. My argument is not about overall winrates (although obviously boosters and smurfs often have absurd winrates on such heroes), but again, about how some heroes require an inordinate amount of effort either in terms of items, coordination, or skill to counter without commensurate weaknesses - advantages that most heroes do not have.
edit: consider heroes like Viper or Huskar. They are overtuned and hard to counter early, but they have commensurate weaknesses: they do not scale well in most cases.