They are fully compatible, they just aren't officially listed as "supported" by Adobe.
This is nothing new for Adobe or for Premiere. User's building powerful systems and sharing tips on this forum have been suggesting builds with GTX cards from NVidia for years.
...the 1070 and 1080 are two of the best GPUs to use for PPro !! Adobe no longer tests all the different video cards and the current version of PPro CC will accept practically any NVidia GPU for CUDA acceleration automatically.
The super expensive Titan X is the top NVidia GPU,but, the soon to be released 1080ti is expected to possibly top the Titan at a lower price, between 8 to 900 bucks.
FORGET AMD for now.....only NVidia cards provide CUDA acceleration which FAR outperforms the "open CL" on AMD cards.
Quadro GPUs are very expensive and are recommended only when there is a need to have a 10 bit monitor connected to it. Otherwise, the GeForce GPUs out perform them.
If you want to see your GTX 1070 fully utilized you have to have a timeline that is filled with GPU accelerated effects. My Premiere Pro BenchMark (PPBM) does that with one of the four timelines, the MPEG2-DVD timeline with GPU. Here is my EVGA 06G-P4-6163-KR GTX 1060 SC and I have overclocked it to 2400 MHz running at 99% usage