Hello Anony

My answer is probably too late for you, but it might help someone else...

Briefly:
AMD Ryzen 9 3950X with Noctua NH-U12S = OK (without much overclocking!)

More detailed:
Actually, it's relatively simple. Noctua has the info on their homepage. The following applies to the NH-U12S:
Specs: https://noctua.at/de...
Max TDP: there is a link to: https://noctua.at/de...
For a CPU with 65W TDP like a R7 3700X: best turbo/overclocking margin!
For a CPU with 105W TDP like 3800X, 3900X and 3950X: medium turbo/overclocking headroom!

Of course, the Noctua NH-D15 Chromax would be even better!
But the fan dimensions and the price are then ... "a bit bigger". (01.04.2020: 120.- vs 88.-)
In return, Noctua gives it a: best turbo/overclocking margin for the Ryzen 9 3950X.
(https://noctua.at/de...)

Greetings

According to official information from AMD, rather not. Water cooling is recommended, here you can see the official recommendations:
https://www.amd.com/en...
I initially had the NH-U9S (smaller Noctua) on it and the temperatures were borderline, not tested at full load. I don't think the NH-U12S will get enough heat away under full load either.