If by "capability" you mean not supporting some latest standard that provides marginal benefit over established solutions (e.g, Wi-Fi b/n/c/whatever, "cloud printing") or anti-features (Apple's "Pro" touchbar), then yes. But if you mean being able to do anything actually useful, I'd say that maybe only those working with Audio Processing still are under-served by Linux Desktop - and even that will change soon with Pipewire.
No. The point of the discussion was about hardware support. Wacom tablets, USB video game controllers and custom editing panels (like the ones for DaVinci Resolve) are AFAIK supported on Linux just fine.
You are really grasping at straws here. We are talking about Linux support for devices in a traditional work environment and I can seriously say that has been more than adequate for more than 10 years. How many backward-incompatible API changes does Apple push every couple of OS versions?
How many dongles were people forced to buy to continue using whatever peripheral they had already working?
The position was quite clear: Linux is more than capable to work as Desktop or business computer for the most common industries and that its support for common peripheral devices has been pretty much in "just works" territory, provided these peripherals are not using or dependent on extremely new protocols or standards.
Good we are all in agreement then... Windows and linux are both at the very least in the "just works" territory. But we also all agree linux is inferior because you caveat-ed your point with: "provided these peripherals are not using or dependent on extremely new protocols or standards." Windows does not generally have this issue.
Linux is, by your own logic, Inferior from a usability or "just works" standpoint. We all agree on this, therefore: End of discussion on this baseline.
Where we disagree is the extent of this incompatibility. Linux IMO is even worse then the baseline above. On linux, my graphics card (arguably a peripheral) has melted down and crashed the OS on me many times. I have a lot of trouble getting my xbox controller to connect with bluetooth to the OS. So anecdotally I would say that even your base line of linux just being inferior is too high.
That's not the end of the story because these small issues extend way past peripherals. You can keep the argument focused on peripherals if you want but this is really a general issue affecting linux and FOSS.