On my latest run through I just found a fabulous place for a Plains farm, so nice and flat, lovely view. Ha, there was just one small kind of unusually flattish looking rock in the center that turned out to be a huge rock the size of a large copper deposit. I had to dig it out then refill (yes I could have moved the farm but had already trenched all around it) Then I got to fill it all back in with the hoe using a combo of flattening and filling. I still get full frames there, only had a few dimples and it turned out much better than I thought it would. And the plants don't die from having rock hidden underground under them. But it took days to do. Tricky devs.

tldr: Before committing to a farm spot in the plains now, beware the flattish rocks. They are like icebergs.