I have not had a good experience with the ASI 120MM on Linux, even with compatibility firmware flashed to the camera. I can confirm it works better with exposures over 1s. But all too frequently I get corrupted image frames. I have even attempted a custom kernel with a modification to the USB driver core. I can not say if that works however because I have not resolved issues with installing the custom kernel.
If anybody is interested in attempting that here are the code changes to the file config.c found within the source files for USB core driver.
I have the same experience. The ASI120MC never really worked for me even with the compatibility firmware installed. I created custom kernels in exactly the same way and managed to install them but gave up in the end due to system instabilities. I purchased an ASI120MC-S (USB3) and that has worked flawlessly for nearly a year now.