I've only tried Ardour a bit, but the first word I'd use to describe it is: Janky.
Been a Reaper user for 6+ years now, after many years on Cubase and a stint on Ableton Live. It's perfect.
How long ago did you try Ardour?
Ardour's development is very, very active, and it regularly gets mountains of features added to it every year.
Ardour can also run LV2 plugins, which is another plugin format. It's not as popular as the VST format but gives Ardour users even more options.
So, yeah, I wouldn't say that plugin availability is a limiting factor for Ardour, any more than it is for any other DAW.
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).