I use mecrisp on STM32s, but when I tried to write it to a Teensy LC, it bricked it. I'm not sure of the relationship between the uC and the bootloader chip, so I'm guessing something in there didn't take kindly to it. Anyway, because it killed the LC, I backed off, and only use Mecrisp on STM32s, where it's been a wonderful dream to use. The nice thing about mecrisp is that it's an optimizing native compiler that runs on-chip, so it's effectively as speedy as C code if you pay attention to what you're doing.