I would have to strongly disagree.  I was playing on endless.  I think I had around 10+ heroes or so in my empire.

I know the dev team has twiddled with this before; I think the mechanic in the way they level is generally just broken.

Maybe a cap on the number of heroes you can get?  Maybe a tech that would increase the number?  Perhaps making heroes die?  Get rid of them from the marketplace?  Perhaps make their level ups exclusive from one another (ie, you choose one, you don't get the other).

I quite litterally could not finish the game because I knew every second turn I would have to open up a hero wheel and click random X level up that really no longer applied to anything he was doing.

I play normal/slower games, and it feels as if the leveling curve is the same no matter the speed. It feels fine with quick games, but longer games it gets atrocious, especially since the academy quest doesn't close the academy until much later so you can end up sitting on 10 heroes or something. You also seem to have the same rate of getting a new hero.

I've found this an issue as well in my games. By mid/late you get heroes leveling left and right, and rather than it being exciting, I've felt it as ending up tedious.

I agree that the heroes should have a harder time gaining a level the higher they get, so that making these choices of how you want to build them and which last tier skills you pick matters. Usually when I hit the last tier, while they can do a lot to impact, I don't feel that I cripple myself much by picking the wrong skill, as the heroes will quickly enough gain another level in the few following turns.

I want to feel that each new level for a hero, all the way to late game, to be an exciting thing to see. Rather than feeling a mixture of "Oh no, not this again" and "I should probably try pick the right skill". I never felt this was much of an issue or feeling I had in Endless Legend with their heroes. Perhaps cause it had linear skill paths rather than freely being able to pick anything, but I feel that wasn't the only reason.