The first thing I do with the "Merge Tabs and Apps" is to disable it and use the in browser tab switcher.
I usually have many tabs open and I usually have many apps open. Merging the two lists just creates more noise in the app switcher. Imagine if they did this on desktop and suddenly alt-tab gave me (right now) an extra 26 things to switch through! I don't even think I am a heavy open-tabber.
My non-techie spouse wanted me to change it back immediately after the update with no prompting from me. I'm sure some people must prefer it but I find it way too busy in the switcher.