Of course having more than 6 or 8 tabs open in a horizontal tab bar makes tabs difficult to manage and track. I've used Arc for the past 6 months. Having 6 or 8 tabs open in a vertical tab bar is also difficult to manage or track. I end up just spam-closing all of them and starting fresh pretty much every 4 hours anyway.
Here's the two arguments I've heard that I resonate more with, one in either direction.
(1) Count the number of pixels dedicated to the tab bar space when it is horizontal versus vertical. I've never seen a functional vertical tab bar that used the same or fewer pixels than a horizontal one.
(2) But: Monitors almost always have more horizontal pixels than vertical pixels. So, actually, a vertical tab bar better-leverages the aspect ratio your monitor is built at. This feels true at 16:9 and greater aspect ratios; it feels untrue, to me, at the 16:10 aspect ratio; and unfortunately, this is an extremely common aspect ratio as its ~the aspect ratio Macbooks are made at.