I was asked a few weeks ago about this piece of material and how I find it in the classroom. I apologise for the delay [Little-N was sick, new term began and laptop isn't working]. I haven't actually had any problem with material and all the children seem to find it easy and satisfying to use. The subtraction strip board i have is the old fashion one where there is one strip and only one place to put your strip. I find the old board a lot easier to use then the one in the picture above.
Your result with the subtraction strip board, relies a lot on your wording. I make it clear for the child that in 7-3, we have to first make 7 on the board by covering all the numbers after 7 with the strip, then we place the 3 strip to take away and we count how many are left on the board. I honestly haven't had a problem with it and I see it as an important material needed in the steps of making subtraction go from the concrete to abstract.
Hope this helps and so sorry for my delay.