Larry - I had a lot of problems with the mini-blind markers fading, cracking/breaking, and even disappearing from my garden, but I have yet to have a single knife pulled up. In fact, sometimes I have a hard time pulling a knife up to check the label because they slide in the ground so tightly and can go pretty deep compared to other homemade markers. I have a lot of squirrels, my 2 pet dogs, and my husband who some times step in my garden beds (grrrrr ....) but the knives have really stood up and they do hurt if you step on them, so I haven't had too much foot/paw action running through my gardens since I started using the knives.
At least that has been MY experience..... I have tried all kinds of homemade markers and these have proven to work far better than I was expecting when I started using them. They are here to stay in my garden!
These are some of my knife plant markers in one of my seedling beds:
