His peers I mean guys around his age or younger.
But is that a qualification? How do you measure him in terms of Rafa, Novak, Stan and Roger, for example?
(Remember we are talking about clay only) I measure him against Nadal, objectively, by his H2H on the surface, which is 3-7. Actually his win percentage on clay against Nadal is (marginally) better than Djokovic's, so that's a huge distinction.
Here is the thing: You have by far the greatest clay court player still playing, still winning. Then comes a guy from a generation which barely wins anything, and he becomes one of the very few players who have a shot against a healthy, well playing Nadal on clay. It is beyond my grasp of understanding how can people don't give credit to that. Ask yourself, or any Nadal fan, against who you can be worried before a match (on clay). You will think of only two names -- and in 2017 and 2018 only one. It is a night and day difference from the rest of the field.
Also, he is 2x2 on clay against Djokovic, 1x1 against Wawrinka, 1x0 against Federer. Just to have a full big 3 + 1 +1 club, he is 1x0 against Murray.
My final opinion, however, is always based on what I see in court. And in court I see a monster of a tennis player (remember, on clay). And the numbers agree with that. Compile other guy's numbers against the big 3+1+1, and see what you get.
Of course, "peak" Federer, Djokovic and Wawrinka are likely in a level above what Thiem can (for now) display (even if the 2017 Madrid final was played at an absurd level). But remember peak Djokovic and Wawrinka came just later in their careers, so it is fair to give him more time to develop.
Off clay, he is "merely" someone close to top 10, sure. But, again, what I see in court is a player trying to change his game. His backhand is already perfect for all surfaces, he is trying hard to add more drive to his forehand. if this will get him there, it remains to be seen.
And I fully, utterly and completely disagree that he is more effort than talent. The effort is there, for sure. But that backhand is pure talent. To be able to withstand a full war of attrition with Nadal on clay is a talent in itself. There is a reason why everyone acknowledges his level on clay and no one ever called him a grinder.