Just to throw my 2 cents into the mix. I received a Flex tuner today and have it set up. It's doing a fantastic job with the ATSC 1.0 stations, including a station with a transmitter 100 miles from my home. While I have a decent sized yagi, it's hanging inside my garage from the ceiling, and most stations signals are coming in through a portion of the home structure.
Problem: while Seattle has seven ATSC 3.0 stations on the air, right now JRiver is seeing all of their ATSC 1.0 signals but one. King TV (virtual channel 5, RF channel 25) transmitter is 26 miles away, and I could probably see it from my home if it was lit up better. I can pick it up on most TVs with only a paperclip as the antenna. JRiver is seeing 5.2, .3 and .4, but not .1. When I use the HD app on my Shield or Roku, the box does see 5.1 and it's viewable on those apps. Of course, the only device that gives me audio on the ATSC 3.0 channels is the Roku Ultra 4800, since it supports AC-4 and sends it to my AVR as 5.1 PCM.
OK, before hitting post, I did some more searching. It seems that JRiver placed it down as channel 61, which of course would be an unknown local channel so there was no data and it was so far down the list I didn't notice it. Once I edited the channel list and reloaded the channel guide, it was all there. No idea why that bogus channel number was assigned, but it's all good now.
If I can provide any assistance with your ATSC 3.0 development, please let me know.