The issue doesn't seem to be related to maximum throughput, its more about coverage/range. On my UniFi-controller the reported signal strength on my iPhone Xs Max when in the bedroom is reported as being noticeably lower compared to my iPhone X.
When located closer to the access point it doesn't have any noticeable issues. It connects to the 5.0GHz network and gets expected 802.11ac throughput. But the connection/signal drops off way earlier and it starts to struggle with the connection in areas where other devices don't seem to have similar issues at all.
Take my Apple TV 4K / 5.Gen also located in the living room, connected to the same access point is having significant better signal strength and is getting much better throughput as a result.
Seems like its much of the same on 4G/LTE as well. Throughput is great, but the signal strength in more challenging locations seems to be quite a bit worse compared to the iPhone X so it will drop the connection in areas and situations where the iPhone X simply didn't. During my daily commute on the train through tunnels where the iPhone X rarely dropped connection my iPhone Xs Max seems to drop consistently.
As the issues seems to be with both WiFi and cellular, which are two different chips from different suppliers (Intel for cellular and Murata in collaboration Apple for WiFi) it seems like it has more to do with the new antenna design and/or gain and not the chipsets themselves. Hopefully the antennas can be optimised and tuned through software and firmware updates-