Surprised even the paid version of Resolve on Linux has no H264/265 support. (On Windows I think you need to install a $10 codec from the Windows store for H265). I briefly tried the Linux version and remember the H.264 limitation, as well as the UI not doing hidpi scaling very well (despite being a heavily modified version of Qt5).

More recently, Resolve also couldn't handle 10-bit video clips unless you pay for Resolve Studio. Kdenlive and pretty much handle anything ffmpeg can deal with.

Never tried Screenflow. I really loved the old version of FinalCut before Apple fucked it up. That was also around the time of the 10.7 Lion release of macOS, which removed workspace rows, crippled expose and was generally shit. MacOS 10.6 was the last good version of the OS and most of their tools.