If fiscal policy takes the lead in stimulating the economy, this can result in a buildup of government debt. If monetary policy takes the lead, this brings about a buildup in household debt. In both cases, stimulus leads to a buildup of debt over time, whether public or private. And excessive debt levels create a vulnerability, making the economy less resilient to future shocks.
Very low (or negative) interest rates for too long raise fears that they may be a source of financial risks by fuelling asset price misalignments and endangering the profitability of financial institutions.