Which makes it unsolvable.
Quite true. You can make the assumption that the icon with one horseshoe would be 1/2 the amount that the number represented by two horseshoes would be, but that's just an assumption. Same with the two boots/one boot.
In truth since the iconography changes the problem itself cannot actually be solved without additional information about the value of the two different icons which is not provided.
Make the same assumption about the horse. One horse does not equal ten. One
side of a horse equals ten. Or does it? Maybe the
right side equals 7 and the
left side 3.
So let's apply that to the following:
2 + x = 7. Solve for x.
According to your logic, this, too, is unsolvable.