Deflation IS a problem if all your money is DEBT, that must be repaid.
ExactDeflation is merly impossible for a debt base currency
That isn't technically true. If the reserve bank issues out less debt than what is expiring. (aka getting repaid). The total monetary supply will be contracting. If the total value in the economy is not contracting at the same rate there will be deflation.
This is why deflation is such a bad thing in a debt based federal reserve system. If such a system enters a deflationary period; everyone has to fight each other for the remaining money to pay off their debt. (thus destroying the money, and removing it from circulation).
Since the amount of debt is significantly larger than the available money. This will force many many people to sell, and that will take value out of the productive economy. A $1M debt is $1M no matter how much $1 buys.
With bitcoin... If bitcoin goes into a deflationary period. When debt is repaid, that money is still free to used again. (not destroyed, like with debt-based fiat). Additionally, the cost of experiences are generally liquid, when the value of bitcoin goes up, the cost of expected expenses goes down proportionally. This is the key reason why deflation has no (negative) effect on bitcoin.