I have a couple of these type of seam rippers. One with a handle and then one that does not have a handle. I have found that they are not all equal. I have found that the one that is a curved blade with the handle (made by Nifty Notions, I believe) does not have a pointed tip and tends to cut blocks. I'm not sure why. My plain blade has a very pointed tip and does not do this and I am able to get even the tiniest stitches with no problem.
If I'm taking out a seam I make sure to pull the seam open, cut on or two threads to get started and at a slight angle push away from me while holding the seam open. If I am taking stitches out of a block I make sure I am holding the area taut so that I can get right under the stitches. Sometimes having a little slack in the fabric can make it easier to slit the fabric.