The R9 3900X runs around 170W overclocked.
If you're using an closed loop AIO (Corsair H100x , EVGA CLC , NZXT Kraken or some other sort) without airflow over the board it's going to run hotter. It should be manageable but run hotter. An air cooled system is going to be thermally limited at the CPU-side when any form of overclock is applied (NH-U14S hits ~95 degrees Celsius per 3dnews.ru ; 91 degrees per kitguru on 240 CLC when overclocked), but also provide more airflow to the power delivery at 1300+ RPM speeds. At stock (no PBO or manual OC) it is going to be software limited to around 140W regardless.
I would expect your system to have adequate airflow if you have two 140mm intake fans and one 140mm basically right over the power delivery. Usually exhaust case fan is around 1000-1200RPM unless it's a 120mm, then you see 1500RPM variants. The Steel Legend is decent about not completely covering the power delivery in plastic (MSI X570 Pro Carbon did this) but the fin area is low.
For the X570 Edge / Gaming Plus / X570-A Pro I wouldn't bother with those on a Ryzen 9 , neither would I attempt it on Asrock Pro4 and Phantom Gaming 4. The minor cost savings isn't worth the gamble.
Another thing to note is all 8 Vishay powerstages on Steel Legend and Extreme4 seem to share the same VRM heatsink so it doesn't have the advantage of more surface area.
The Steel Legend is well balanced for R7 3700X features-wise, I haven't seen it being run with R9 3900X yet. There's a few Korean reviewers that pushed R7 3700X on X570 Steel Legend.
It used to be Steel Legend was a B450 / locked Intel tier board in the product stack but Asrock's presentation for X570 has it as mainstream along with the Extreme4, with Phantom Gaming 4 and Pro4 as "value" / budget boards. Recently Asrock also launched the Z390 Steel Legend (around April) but that was well below the Extreme4 in terms of power delivery so they're definitely moving upmarket with that branding.
The reason why Aorus Elite is the de-facto choice at the moment is because it is the cheapest board on Siliconlottery's QVL and the power design is basically that of the $300 Asrock Taichi / Asus STRIX X570-F (with phase doubling). See https://siliconlottery.com/pages/amd-matisse