Often times when I bench with an arch, I get unusual pain in my lower back. Whenever this happens, I pretty much stop flat benching. (This doesn't happen on incline due to no arching or leg drive, which, from my experience, is just there to maintain your arch.) There was a time in 2020 where I hurt my lower back so much, I randomly couldn't even stand up. I was in so much pain. Of course, the doctors thought I was bullshitting and didn't prescribe me any pain medicine. This was from doing lots of flat benching with an arch at my home gym during covid. I'm so glad I've stocked up on 7OH for emergencies.