Having true damage, %hp damage, or stealth does not make a champion brokenly. Generally most of the first 2 are fine (you don't see many people complaining about Twitch's passive or Sion's W) and the only problem with the third is the current power perma stealth has (which is thankfully only on Eve).
If we're going off of rates though, Heimerdinger has a 53.7% winrate in plat+ ...but also only a .94% playrate and .22% banrate. His winrate is high, but that's because the only people who pick him are die hard Heimer mains, who still only pick him sometimes. He's niche, and when picked into that niche is strong but suffers everywhere else.
On the other hand, Riven has a 52.68% winrate in plat+ ...and a 16.84% playrate with a 9.71% banrate. Her winrate is high, but she's being picked very regularly everywhere and still winning. That indicates she is somewhat strong (though whether that's because she counters the meta, has a high skill ceiling and ways to express skill, or other I do not know).
For a comparison, out of 1 million games (1,000,000) you will statistically see a Heimerdinger picked or banned 14,600 times. You will statistically see a Riven picked or banned 265,500 times, or roughly 18.2x Riven pick/ban per Heimer pick/ban.