The fact that that the original residents are displaced to a place without those benefits and bear the cost of relocation, while the people that enjoy he benefits are people who could have afforded to get as good, or nearly so, conditions in existing places with them, anyway.
Gentrification is a magnifying downward redistribution of misery, where the new residents get a small gain in exchange for the old residents (particularly renters) getting a big loss.
Yes, property owners—who tend to be more wealthy than renters to start with—may see benefits. That kind of reinforces the point, rather than contradicting it.
What exactly is bad about it?