Eutychianism states that Christ's natures were so thoroughly combined - in a sense scrambled together--that the result was that Christ was not really truly able to relate to us as humans. This is similar to Monophysitism. The problem is that this implies that Jesus was not truly God nor man. Therefore, He would be unable to act as mediator and unable to truly atone for our sins. (For more information, see Hypostatic Union and also Heresies).