what is the problems of monetary and fiscal policy?

Answer 1

The previous answer addresses some of the issues.

In addition

Fiscal policy - in order to spend more the government must do either of the following - tax more or run up a deficit.
By taxing more, the government reduces the amount of money that consumers are spending or investors investing. Each will hamper growth. The Balanced budget multiplier theorises that a government can tax higher, spend more and create growth by spending to a macro-plan that will produce large benefits. This of course relies on the government having such a plan and being right. Running a deficit will involve the government borrowing and so increasing the demand for money. Interest rates go up and so investment is discouraged.
Monetary policy has a couple of issues. Other than previously mentioned the biggest impact is that it tends to emphasise the boom bust cycle. As an economy recesses, interest rates are lowered, and investment and non-investment borrowing increases. This strengthens the economy but is very hard to balance and suddenly we find the economy overheated. So interest rates are increased. This causes hardship on those with debt and can often hit the poorest the hardest.
Of course when an economy is overcapitalised (as for instance the US was in the great depression), increasing investment opportunities will have little effect as companies will not want to invest in new plant, when already plant is sitting idle. As our economy becomes more complex, more and more investment is speculative rather than expansionairy and so the effect of monetary policy is reduced.

Answer 2

From what I remember from Macro, each has one big problem. The problem with monetary policy is that changing the money supply only can achieve on target at a time. In fiscal policy the major problem is the time problem. For a government to spend money to increase the money supply it takes time to pass laws and spend the money. This in turn means that the government’s policies may be out of date by the time their solutions are implemented.