Yeah, it looks like expected and have alot of bugs, as espected
Right now, "tabs everywhere" is a huge stretch, some apps, like Mail, dont even seem to have the support to create tabs, i had to create 2 separate windows, then use "Windows - Merge windows" (or whatever the translation to english really is), then i got 2 tabs in Mail (Draft and Inbox on my screenshot), does not really work that well, one annoying bug is that even though i now have 2 tabs in mail, the both tabs are actually located on 2 different "Desktops" so when changing tabs, it also uses mission control, or whatever, to move to another desktop, you have to move both tabs to the same "Desktop" manually to not have the tabs moving to other "Desktops", so visually, they are contained in one window, but those windows inside the tabs actually works like separate windows behind the scenes
I could not get tabs to work in Calendar, tabs work great in Maps.
Problem is, apps that have used tabs for a long time, like Safari, Finder, Chrome and everything else, those have "new tab" on cmd+t, problem is, alot of other apps have the cmd+t already assigned to something else, like Mail so what will Apple to to get a coherent systemwide tab shortcut? Force other apps to let go of the cmd+t so Apple can use the shortcut systemwide?
Let me know what apps you want me to test or screenshot