It doesn't seem possible to easily write your own plugins or extensions in Ardour? Which is rather trivial to do in Reaper. Also, Reaper automation and customization is limitless.
But, Ardour is free, Reaper isn't, so that is a big difference.
But, Ardour is free, Reaper isn't, so that is a big difference.
I wrote some plugins for fun a few years ago in Lua: https://github.com/robsco-git/ardour-plugins
My understanding is that the plugins one writes for Reaper are compiled on the fly so I imagine they offer better performance.
You could just use any of the thousands of VST plugins out there, which Ardour fully supports.
But sometimes the thing that you need does not exist. I needed a "MIDI notes compressor" and couldn't find one, so I wrote it (the compressor brings all notes to a chosen range, so for example if the range is A3-A5, a C7 will be converted to C4).