Safari 15 is a beta software, but I love new tab design on all platforms, especially iOS (completely got used to tabs on the bottom and one handed operations are very convenient and quick now).
I have to say though - first thing I do is turning off Tab coloring, since that looks completely out of place. Takes courage to make it a default look for sure.
As many mentioned - sometimes a wrong tab can be closed though (especially of tons are opened), since there is not enough shading on the current / open tab.
That is, the only thing that really lacks is a bit of tweaking design wise of the active tab shading. Various shading grades and even colors should be provided so that everyone can find the right setting.
Since there is a way to get back to the old tab design, there does not seem to be any other problem design wise.
IMO: favorites above tabs just look so wrong though. Especially on the screenshot above. But well, if somebody likes it that way - an option to choose tabs bar below favorites will make those users happy too.
Apple Designers, just give options to users in settings. Apple programmers, just focus on stability which is badly lacking at the moment. Crashes, inconsistent sync of Tab Groups - it has been a rough ride with Safari 15.