Thanks, this helps and I vaguely remember the issue with limited interrupt support (in the K66.beta thread?)Two reasons:
Teensy 3.1, 3.2, 3.5 have only 16 channels.
Teensy 3.6 has only 16 which support unique interrupts. The way priorities are assigned is also a 16 kludge, not really 32.
Someday I plan to add an extension to DMAChannel.h to allocate 16 more without interrupt support. But that's a very low priority, where "very low" probably means "unlikely to ever get around to it".
What's *not* going to happen is 16 overhead-adding dispatch functions like those used for attachInterrupt.