The upper management are always the one to invite the show, usually when they are on the brink of bankruptcy- I don’t think the employees have ever been (or been represented) as calling in the show. Without exception, the upper management/owners are the main problem. Most of the time it is because they’ve become complacent (have a more hands-off role) or lost touch with reality (like, their food sucks). Sometimes it is because they’re just fucking idiots (one guy bought a Danish restaurant, and of course the food suffered, because the guy who bought it wasn’t Danish, and he didn’t want to change the menu).

There are often problematic employees, and oftentimes the worst of them get fired or quit because they have been found out to be hacks (usually the executive chef). But most of the show focuses on changing the owners and how the place runs.

People react in all sorts of ways. Sometimes people and places have a genuine turnaround (you can see the turnaround in yelp reviews, before/after the show’s taping).
Most of the time after the show airs it doesn’t make a difference- the owners change for a while but revert back to their ways over time.
Sometimes the owners react with extreme hostility. They’ll literally change everything back to how it was within a week of the expert leaving, and are yelling the whole time, and refuse to acknowledge what they do wrong (always blaming the employees).

So it really depends on the person. It’s probably worth finding an episode of Kitchen Nightmares (the older the better) and checking it out, you really only need to watch one to imagine all the permutations.