Central bankers currently offer quantity targets in these cases – a volume of assets to be bought per month – but price targets are only given in general terms, for instance “flattening the yield curve”. This is appropriate so long as balance sheet policies are temporary.
It may not be in the long term, but moving to price targets may come at the expense of price discovery. This links to the second complication: balance sheet policies can mitigate certain facets of the safe asset shortage, but they might exacerbate others. In particular, balance sheet policies have been stimulative by swapping central bank money for assets with duration and credit risk.