Even worse putting it all in one cell still evaluates to 0. `=128^128+1-128^128`

This is because Google Sheets is using something similar to double precision, otherwise mathematical calculations could easily halt the average computer. 128^128 is an awfully large number and if you need perfect precision with numbers that large a spreadsheet is probably not the right place to be doing your calculations. Remember as of today the limitation of google sheets is 50k characters per cell so that would also limit your ability to do amazing calculations.

using the base function we can see the actual accuracy of numbers... `=base(128^128+1-128^128,10,1)` yields the following error: 

    Valid values are between 0 and 9.0072E+15 inclusive.

which is a fancy way of saying if you want to work with even whole numbers above 200 trillion with exact precision you shouldn't be using normal spreadsheet software.