USB Bootloading: ATmega32u4 vs SAMD...

So I have some SAMD21 and a SAMD51 board that drive me crazy - normal "upload" fails about half the time, and frequently the serial port disappears entirely from my system, requiring a power-cycle at the USB Hub level to get it to reappear. These are supposedly "known problems" due to the rapid disappearance are re-appearance of the hardware on the USB bus, and there are known solutions that work most of the time - double-tap reset to stay in the bootloader, make sure you don't leave the serial monitor open, and try to time the serial re-connect after upload "just right." And I've just "lived with it."

But I was doing some USB speed testing recently (8Mb/s Serial.print on the M4!), and pulled out an old Leonardo board to compare. It seems to be MUCH more reliably for uploads and rebooting than the ARM boards!

How come? Overall, they should have the same issues, and it really ought to be possible for the ARM boards to work as well as the 32u4...