It's not all black and white, at least from my experience
A similar thing to what you described happened at a software company where I used to work at, culture destroyed, many people let go. I will name and shame the PE firm - it was Hg Capital
However currently, I've been at a company for a few years who is owned by Morgan Stanley Capital Partners, and it's a completely different story. The culture is great and hasn't changed at all
A previous firm I worked at was bought as part of a roll-up (market segment consolidation). If you're the firm that they're rolling all their acquisitions into, that's great & exciting. If you're one of the roll-ees, not so much.
They bought us not for our technology but our customer base. They intended to convert them all to their other firm's product. Little did they know that a lot of our customers had left the other firm for us because we treated them better.. So what happened is in addition to the back office staff & sales staff being laid off, they laid off the developers & testers too (they kept a few managers for a year for continuity). I realized this when the folks they sent to town refused to go to lunch with us in an rather awkward moment.
That is funny as the exact thing happened in my startup as well (we were one of the roll-ees)
We got some suits sent by the PE after the funding round.They politely said that they had other plans when we invited them for lunch.
A similar thing to what you described happened at a software company where I used to work at, culture destroyed, many people let go. I will name and shame the PE firm - it was Hg Capital
However currently, I've been at a company for a few years who is owned by Morgan Stanley Capital Partners, and it's a completely different story. The culture is great and hasn't changed at all