Why is this iPhone software so buggy?

We upgraded our 4S iPhones to 9 and the problems are awful, bad sound, battery life forget it, and one phone has no WIFI at all. Went to Apple store they told us to get a new phone or come back tomorrow because they were busy. Have talked with support twice, done everything they said to no avail. What good is an iPhone without WIFI? And the iPhone also keeps loosing the Bluetooth connection. None of this happened before the upgrade last week.

But so far no help from Apple has accomplished anything ...