What makes analyzing an algorithm on a real computer complicated?

$$ A real computer executes instructions one at a time, but to analyze an algorithm, it is necessary that the instructions be executed in bundles or packets.

$$ A real computer's memory does not have the ingredients that make it possible to analyze an algorithm.

$$ On a real computer, it is not possible to take into consideration the type of compiler used when analyzing an algorithm. This is always possible when using a virtual model.

$$ A real computer executes instructions in bundles $($pipelining$),$ but the virtual model executes instructions one at a timefit