I updated my iPhone Xs Max, my wifes iPhone Xs and our iPad Pro 12-inch 2.Gen and the results according to our Ubiquiti UniFi-controller is basically the same. We have 2x Ubiquiti UniFi AC HD (Qualcomm-Atheros QCA9994 chipset) access points and the results are pretty consistent.
The issue with iPhone Xs and Xs Max isn't throughput. Its fully capable of maintaining great 802.11ac RX and TX rates when located in short proximity of the access points. Speed test also verifies that its fully capable of using our 500/500 mbit bandwidth. There are no real issues there.
But the signal strength is disturbing even when located in close proximity. In my computer room sitting just 1 meter from the Access Point it's reporting a signal strength of 79% (-59 dBm). Our HomePod in the same room is reporting 99% (-47 dBm) and the HomePod in the hallway a few meters away is reporting 97% (-52 dBm). My Apple Watch Series 4 is reporting 84% (-43 dBm) but the Apple Watch doesn't seem to be supporting 5.0GHz so it's connected using 802.11n on 2.4GHz with 40Hz channel width and not to the 802.11ac on 5.0GHz with 80Hz channel width.
It becomes worse when you start moving away from the access points. The iPhone Xs and Xs Max drops the signal quality and strength much more rapidly than any other device which is being confirmed by the UniFi-controller.
It's not that much of a problem as we have stellar coverage at home with 2x UniFi AC HD access points. But at work for instance with a single Aerohive Access Point I notice that my iPhone Xs Max keeps dropping the connection when moving towards the toilet and coffee area of the work space. My iPhone X would lose connection when entering the toilet at times, but my iPhone Xs Max is dropping it consistently.
It's the same for LTE/4G, during my daily commute my iPhone X would sometimes drop connection, especially through tunnels while on the train. But my iPhone Xs Max is dropping it consistently when going through the tunnel, without exception where my iPhone X would do it seldomly. I also notice that it seems to fall back to 3G a lot more compared to the iPhone X.