In which case might the number of comparisons NOT be a good representation of the cost for a sorting algorithm?

When we are comparing strings of widely varying length
  • When there are lots of records
  • When the CPU is really fast
  • When the amount of available space is small

CPU speed would affect all comparisons in the same way.

Number of records or amount of space won't affect the value of counting comparisons.

The longer the string, the longer it takes to compare.