1. Great if you have a wider screen (could never do it on my old 13" Macbook Air, for a 15" it's pretty good but for a 24" iMac it's perfect). But if you need the space youjust have it set to minimize by default, maximize on hover.
2. See the titles of your browser tabs, which is great when you are like me and never have fewer than 30 tabs open at once.
3. Easier to select browser tabs when you have many of them open (ie they don't get squished unreadably small)
They really only work if you have a large monitor. I use a 50" 4k TV and two other monitors, and a 15" laptop. When I'm on smaller screens I have to hide the tabs.
I find the Tahoe tab bar pretty ugly.
Pretty happy with it; I tried basically all browsers out there, fully switching to them for some time even if I didn't even like them, and after all that time I found Vivaldi the best overall browser right now (for me).
It's a shame, because I like the attitude and spirit behind Vivaldi.
Of course you can customize with CSS and other types of things, but I'd ideally like my browser to just work well and be designed well.
Having said that, I miss some features from Vivaldi. However, I am much happier using vertical tabs in other browsers that have that feature.