IMO it's not generally the fault of the private equity. The company isn't that profitable, the owner gets tired of running it, and someone comes along and says "hey, how would you like it if I just bought this out from you and you can retire on an island?"

The owner is like "perfect, sold", and they buy it from him with raised money and debt which is borrowed against the assets of the company and then they either screw it up and run it into the ground, or else they hire a beancounter to make it overpriced and shitty so it can continue to pay its bills.