Digging beyond the scores, I'm seeing conflicting criticism - often within a single review. On the one hand, it's the most Nolan of all of Nolan's films: Familiar. A step sideways. More of the same, etc.
On the other hand, it's confounding and in need of multiple viewings, compounded by a few slightly more objective flaws (sound mixing) muddying the waters. It's an interesting contradiction (my own immediate reaction to the film was very similar), and it makes me wonder if the film's legacy will more closely resemble The Prestige.
Recall that The Prestige similar confounded critics, with many dismissing it as complex nonsense, while others wrote it off as a fairly straightforward thriller. That film is among Nolan's lowest scoring, but its legacy has appreciated considerably since its release. Upon reevaluation, it's often regarded as Nolan's finest work and the key to his entire filmography.
Will Tenet age similarly? Perhaps, perhaps not. But keep in mind that these reviews are just the beginning. Also keep in mind that confounding factors like the ongoing pandemic may place an unfair downward pressure on scores - particularly in the US (though I don't expect this to make a huge difference).
Likewise, there seems to be an encroaching bias against Nolan as a pretentious, self-satisfied filmmaker. And while Nolan does have a tendency to overreach, he also desperately wants audiences to understand and follow him along on the journey. I think it's an unfair and surprisingly anti-intellectual point of view. But I suppose we all have our own biases.
On a mildly unrelated note:
If I were to quibble with reviews on one point, it would be regarding criticisms over the lack of any real emotional connection to the film's central character. As I see it, cinema is full of great films with unnamed, seemingly underdeveloped leads with mysterious pasts. Films like Le Samourai, Blade Runner, Sergio Leone's Dollars trilogy, etc. Even Bond to some extent, though the quality there varies greatly. Either way, as in all things, it's really the execution that matters.
Personally, I did not have a problem with this cooler emotional palette - nor do I think that the lack of a romantic subplot or unrealistically intimate stakes reduces the film to that of a mindless actioner. Give me a mysterious drifter over a cookie-cutter origin story (complete with a generic love interest) any day. Nolan's noir influence shining right through, there. Not to mention a more graceful fit for the espionage genre.
In a way, JDW is a surrogate. That is to say, his lack of a clearly-defined past serves as an anchor for the audience to project their own subjectivity (a similar approach that worked well for Dunkirk). We learn about this twilight world as the protagonist learns.
Contrast this approach with Inception, where Cobb himself is, to some extent, the mystery. A man with secrets. Someone who already has all of the answers and isn't sharing. There, the audience requires some other reason to empathize with his character. Whereas in Tenet, I didn't feel the need for an additional sentimental subplot, ala Mal (which I personally found to be well-intentioned though somewhat clumsy and artificial in its execution, in spite of Cotillard's performance).
Frankly, I don't believe "emotion", in the more traditional sense of the word (see Spielberg), is something that Nolan is particularly gifted at anyway*. And what Tenet goes for instead (particularly with the JDW/Debecki dynamic) is something a bit more understated, which works better for this particular story. In short, I would disagree that this feature alone makes Tenet an inherently shallow film. To me, that's a very surface level criticism that time will likely prove wrong.
*I did enjoy the dynamic between Coop and Murph (at least in Interstellar's first hour), but that was a different film, with different narrative ambitions than Tenet.
