Difficult to say. Every time I open a world, it works for 5~10 minutes, and then my computer crashes. And I meant totally, so I have to force a reboot. It makes it rather difficult to see if there are any error logs (unless they are saved somewhere?), though I have had the console open on a second display so that I may see if something was happening, but it didn't seem like it. Memory use seemed to be stable at about 1 GiB. CPU usage was quite high though.
I think that is the best explanation I can give.
EDIT: Oh, and I did try the disable mystcraft-etc.-mod solution, but it didn't seem to help. I haven't tried Fix #2 yet, but I will in a while.