I have 15 years of experience running Ubuntu Studio professionally, and 20+ years doing pro audio with other platforms. I'm currently producing/engineering in a high end studio loaded with all the latest high end gear, and I stand by my statement: Ubuntu Studio is clearly a viable, professional platform now.
I've done a ton of work with this, and its a rock solid and highly stable DAW platform. More so than MacOS' recent Catalina clusterfuck .. and light years ahead of Windows' headaches.
Sure, its not the hype-intensive candy land of Windows/MacOS, but then again .. that's the point, entirely. From the get-go, Ubuntu Studio packs an amazing punch - just installed, out of the box, there's more happening in this OS as a DAW than on either MacOS or Windows, and even after a fresh install of either of those options, you'd still need to spend hundreds of bucks on programs and plugins just to match what Ubuntu Studio offers. Plus, go through all the hassles of installation, iLok, free plugin installation nightmares, etc.
You'd be surprised just how great things are, out of the box, if you'd just get over your prejudice about it not being 'mainstream' enough to support your ideals for what constitutes 'professional' DAW usage.
I run REAPER and have tons of VST plugins from my Windows DAW onboard the Ubuntu Studio rig - they just plain work.
Sure, it doesn't get the attention it deserves from pro audio developers - but that is changing. Lots of devs are seeing Ubuntu Studio rise in the scene. And this is only going to improve with this inclusion of KDE ..