Well, I don't know that it's really a "crap shoot" but a 30T may not work for all bikes. Bikes with a longer derailer hanger or thicker derailer hanger may have a higher probability of working since the derailer will be slightly lower when on the big cog. The hub and chain length may also play a role. I have used 13-30 with my Dura-Ace rear derailer but it didn't really run all that smoothly.