Hard carries need a lot of farm before they fully come online and can be truly effective. Many require additional farming equipment such as Battle Fury, Hand of Midas or Maelstrom. They thrive in long games and when they have time to farm efficiently.
They generally have strong passive skills that provide exceptional bonuses in the late game, but minimal help in the early game. They're strong against other carries who want to farm, but weak against pushers and teams with very strong early game.
All of the heroes mentioned here have extremely minimal utility early-mid game - the odd skill, say Assassinate or Chronosphere may net kills or help the team, but generally they are weak at this point, almost regardless of build.
+ Very strong late game
+ Good choice when played with support against solo enemy