Now, I've read about the 536th clickbait headline about #Oppenheimer and why "it might not be a masterpiece".
Long story short: It is.
It weaves together three timelines of very different pace and - let's say - interestingness. I can't imagine how this should've worked in chronological sequence.
Nolan clearly distinguishes between the three by filming style and narration.
The topic in itself is very complex and the film respects that - no added drama, no unnecessary simplification (nor oversimplification). And, yeah, our modern life involves many people. Many characters. The One Man Invented It story is a lie, at least since 1900.
Finally: the acting is 1st class.

So, yeah, critics: If you didn't like it, there are two options. Either it's not your style or you couldn't follow. In the latter case it's probably because you somehow expected big explosions and were surprised by ... plot. A thing that's unfortunately not very common in many recent sequel-based franchises. So, I only blame half of it on you.
If it's not your style, maybe the Arte documentary about J Robert Oppenheimer might suit you better 😉