I use Digispark with two mac's - a macbook air, and a 2011 mac mini. I've never experienced the freezing on my mac mini, but I do not use usb keyboard or mouse - I use the apple bluetooth ones, and I think this makes quite a difference!
On my macbook air during program uploads often my keyboard and mouse will freeze for a few seconds during upload. I'm not sure why this is, but I don't think it's the digispark because it seems to happen when I click the upload button, regardless of if I plug a digispark in or not! Maybe the supplied digiusb send program is using the same code my micronucleus upload tool uses, and therefore has the same issue on Mac? In any case, I've not observed any freezing when using
the rubygem.
It probably isn't useful in your case, but I also want to mention that on my macbook air if i keep wiggling my finger on the trackpad at the same time I use cmd+u to start program upload and plug the digispark in, nothing freezes up at all. Weird, huh? Merely using the input devices somehow stops them from getting all confused seems like.