Wow... where to start?
1. The R9-390 is NOT comparable to the GTX970 if you look further. The new cards are the same GPU as last-gen but are HEAVILY OVERCLOCKED at stock and thus have minimal overclocking left.
Thus STOCK vs STOCK is not comparable as the GTX970 overclocks further. Here's a benchmark (couldn't find a good R9-390 with lots of games):
http://www.techpowerup.com/reviews/MSI/R9_390X_Gaming/30.html
Note how high the R9-390X scores compared to the R9-290X. Assume the R9-390 does that and it should match the GTX970. Again though, the GTX970 also have overclocking left.
2. Better DX12 support?
R9-390/390X-> DX12_0
GTX970/980-> DX12_1
http://www.extremetech.com/extreme/207598-demystifying-directx-12-support-what-amd-intel-and-nvidia-do-and-dont-deliver
Some people where confused by an AMD slide presentation that said "DX12 Tier 3" support which is misleading.
3. Lower level of support for NVidia DX12?
I think I just proved that incorrect, though the advantages of DX12_1 over DX12_0 might not get used much.
4. NVidia poor support for older GPU's?
Proof?
I have a GTX680 and tested over 300 games with NO PROBLEMS to speak of. Most of the community seems to agree that NVidia is now ahead on driver support. Probably due to AMD's financial problems.
Other:
We also didn't discuss power, MFAA, H.265, HDMI 2.0 support, voxel lighting, HUD support, and PhysX.
*So not sure where you're getting your information but you really aren't correct.