Mattress bonds lose value in inflation, I bonds rate are tied to consumer price index
If interest rates are allowed to rise to level commensurate with real inflation, the current bond values will be drastically lower. For example last months inflation rate was over 4%. Lets say FED raises short term interest rates to 4%.
Then a bond selling for $1000 could drop by 20% or more.
Money hidden in mattress will lose only 4% in value.