A random comment by spideywing in IRC made me sit down and calculate up the percentages of UST to death in Kamen Rider shows in the Heisei era. Here's what I came up with (up through 2012's Movie War Megamax):
IF YOU HAVE A SIGNIFICANT OTHER WHO IS A KAMEN RIDER AFTER 2000 - There is a 53% chance you will live. - There is a 35% chance you will die. - There is a 12% chance you will live, but your SO will die instead.
Note that if you just have UST with a Kamen Rider, your chances of living pretty much jump up to 100%. It's only if you're at the dating stage or past while your SO is still fighting that you might die.