It's a three-part problem. 1) Power creep is absolutely a thing that happens---it's hard to avoid. 2) In developing new army mechanics, sometimes they stumble upon mechanics that are powerful but oppressive for the opposing player, requiring a higher level of skill to overcome. 3) Armies that become hard counters to others (something that hasn't really been a major problem until recently).
Going back only a year ago, you can point to many allegiances, abilities, warscrolls, etc that have been further iterated on with stronger rules than their predecessors. Many people tend to agree that books releasing earlier in the life cycle lose their weight near the end. It's natural for this to happen. Designers want to err on the side of strength, either for marketing purposes or to ensure a book/product has staying power. A book with a "safer" rule-set is always going to be harder to lift up than a book that's more oppressive; because the tighter and more reliable the rules, the more readily points increases can mitigate its power (not that this is a foolproof idea, but largely true). However there's only so much you can reduce an army's points to make up for discordant rules (look at BoC or Gitz) before things get a little silly and you end up with 300 models on the table. Power creep happens in nearly every game. The real question is by what method and to what degree?
Are Petrifex, Tzeentch, and Slaanesh overpowered armies that have crept their way into the top echelon by rules-writing alone? It often depends on the player's point of view. I often see very competitive players mentioning that there are clear counters to these armies if you have the right setup and know the right strategies. These folks will obviously say they're fine. Casual players who just wander into the game store every couple weeks and don't pour over tier lists will shout in agony to the high heavens and refuse to matches against them because of their miserable experiences playing against such armies. Nobody wants to spend 2-3 hours playing a game they couldn't win because they didn't do enough mortal wounds or didn't own enough shooting units.
The fact of the matter is people want to buy, paint, and play with cool models. But when playing against some of these armies, you can't simply throw whatever you have/want onto the table and hope to have a chance. Even just owning a mixed army of your favorite models with a couple synergies flavored in there won't cut it, and you'll get smashed a majority of the time. Newer armies tend to skew towards needing a higher level of learning to beat, very specific counters, and they often have multiple safety nets (reliability) for the controlling player to avoid the ruthless nature of luck in a dice-based game.
Why aren't people talking as much about Petrifex or Slaanesh anymore? Well because KO and Tzeentch have released since then and are pretty strong direct counters. Yet people still speak about Petrifex with venom on their lips, because many people aren't playing with those two armies, they're playing with armies that still intensely struggle in that match up due to the nature oppressive things like hyper-defense + offense or infinite summoning. Hell, barely anyone talks about FEC anymore, but I don't know anyone that enjoys playing against them specifically because of the chalice and terrorgheist. Would competitive players say FEC was overpowered? Probably not. I wouldn't. But the mechanics of their army, the very things that give them the best opportunity for success, are oppressive and difficult to overcome without prior knowledge and a lot of experience. I don't want AoS to head in a direction where pitting one particular army against another means an assured defeat by one of those players, but in some cases, we're already there, and it has been getting a little worse.
That said, I'm hopeful the rules writers at GW can turn things around, mitigate some of these oppressive rules, and have the eye to spot a need for change.
TLDR: Power creep happens and it's more prevalent when mechanics become too overwhelming for casual players to handle effectively without specific army compositions or extensive experience.