He definitely is a very good manager (as was obvious before) who nonetheless has some flaws (meaning he is not among the absolute best). In Dortmund there was some trouble but Dortmund is always trouble in recent years, that's probably not really his fault. In the end he has now won two of the three titles Leipzip have ever won (granted the Super Cup yesterday doesn't count for much, but who cares). Im comparison to someone overhyped like Nagelsmann he definitely does a better and more solid job.
I think Leipzig is where he belongs. They offer him a nice pseudo-underdog role, where he gets praise when things go well and no one talks about it when they don't, like going 0-11 in three successive games against City, Bochum and Mainz.
And while Dortmund did have their issues during his stay, there were too many absolutely unacceptable results to even think of absolving him of his responsibility. Like the cup campaigns: getting hammered in Amsterdam, going out of the CL against Sporting, then losing 2:4 at home against Rangers in the EL - and not winning the return leg either. In the domestic cup he went out to St. Pauli, too.
Then in the league there were another bunch of completely unacceptable results, like losing 1:4 against Leipzig, 2:5 against Leverkusen and finally having an embarrassing 3:4 defeat in the derby against Bochum - all at home. 6 defeats in 17 home games, I had to go back as far as 99/00 to find a season with as many losses. Terzic, coaching the team before and after him, has 2 in 28 matches.
I think he's a good coach, but I don't see anything special about him. He's playing the standard Leipzig football, getting about as much as you can reasonably expect out of the squad. And I got the impression that he's not very comfortable with the media. I don't see why bigger clubs should look at him. And I don't see any reason to compare him favourably to Nagelsmann, who actually pulled off some miracles before joining Bayern and whose performance there - in hindsight - have been looking better with every passing week.