I remember using the Unsanity Haxies on my system and a few systems at work back in the day, but remember that whenever I was testing software or had software issues, those were the first things that got removed when something went awry.
I’m looking at FruitMenu and hoping it’s not a destabilizing force any longer.
Classic MacOS had only minimal resource protection, so a rogue app/utility could corrupt the system.
Since Mac OS X (10.0), the Unix kernel and other system services do a great job of protecting services from each other. A bug in a “haxie” could mess up the system service it hooks into, but it shouldn’t be able to destabilize the whole system.