Yes, I know it's frustrating.
In a perfect world, we would have implemented absolutely every feature from Teensy 3.x before releasing Teensy 4.0. The the reality is I'm not that good. PJRC isn't that good. And if you compare to pretty much every other product out there, almost nobody is. Even Raspberry Pi 4 still is features of Pi 3 like USB boot to be delivered in forthcoming firmware updates.
Teensy 4.0 was in private development for well over a year, then a public beta test for about 8 months. We did get a pretty large portion of the feature list working during that time. Also during that time, there was a growing feedback from people watching the beta test that could be best described as "shut up and take my money".
We couldn't keep it in beta another year, not selling any and keeping *everyone* waiting without any access to the hardware. There had to come a point where the beta was cut off and we started to actually sell hardware.
Since then, development has continued at a pretty good pace, as anyone can see from the long lists of changes and improvements on each version.
Also yes, ideally we would have top-notch documentation with a huge feature list or matrix. We had a list during the beta test, but even then that list regularly fell behind. Maintaining that sort of documentation as things are rapidly changing is a lot of work. If I or others who wish to contribute put our time into that, it means hours not going into investigating and fixing bugs, porting libraries, and implementing missing features.
And I am planning to put several solid days into documentation soon. But it's not going to be a moving-target feature list. So much documentation work is much more urgently needed. Near the top of my list is filling in the memory model stuff, which was about 80% written a couple months ago, but lacks examples and more specific usage info (because 1.49 was coming, with the FLASHMEM keyword).
USB audio will happen. I know you wanted that, and it's disappointing we wrapped up 1.49 without it. But please try to understand there's only a limited number of hours in every day. Things can only improve at a certain rate, so time has to be prioritized. USB MIDI & HID, and Wire lib stability on troublesome chips, and the missing I2S features too priority.