First, under yield curve control, it is uncertain whether the pace of the Bank’s JGB purchases will slow to an extent that would be sufficient to enhance the sustainability of such purchases; instead, there is a risk that the Bank might need to further increase the pace of its purchases.
Since “quantity” and “interest rate” are generally determined simultaneously, my understanding is that it is difficult to conduct smooth money market operations while setting explicit targets for both dimensions. When the interest rate dimension is to be controlled, control over the quantity dimension could be lost.