Did you install El Cap clean, or did you upgrade a system that already had CalDigit drivers installed?
I ran into a loop of constant restarts and kernel panics with the El Capitan beta, but only after I installed the Yosemite CalDigit driver to see if it would work. Before that, with a clean install of the El Capitan beta, the only problem was that the USB 3 drive attached to the CalDigit card was not accessible under El Cap. But there were no crashes or kernel panics or restarts. I'm seeing the same stability now with the released version of El Cap.
I have the first-gen CalDigit card and it has been working well with Snow Leopard through Yosemite. Last time I checked, CalDigit seems to have pulled the Yosemite driver; no idea why, since it seems to work for me — but I spend very little time in Yosemite, so perhaps I just haven't had the chance to run into problems with it. I will be annoyed if CalDigit decides not to update the drivers, but it's good to know that the Sonnet card is an option.