There are a lot of technical issues with TRON. I'll be the last person to say that the movie is realistic, but it certainly doesn't have to be "realistic" to be "believable."
The first TRON movie set up an interesting mythology in an age when most of the audience would have understood very little about computers, and their inner-workings.
The story takes place in a world where it is possible for a human being to be translated into the world of software, and to interact with the programs as if they were people with lives and personalities.
Is this a real place, or is it simply a fantasy created in Flynn's head as a way to comprehend what was really happening to him inside of the computer system, events which would have been impossible to describe or depict?
The new movie, in fact, tries to answer that question, as Flynn describes his previous journey into the system.
He describes that, for many years, he had frequently visualized the inside of the system like a world with motorways and buildings, where each program had its own life, and that, once translated into the system, that's exactly what it did look like.
This even explains how his son, the only other person seemingly exposed to these images, is able, once translated, to see the system the same way.
All of that works for me. All of that maintains a sense of continuity which is loyal to the original mythology of TRON. Then, in an odd turn away from verisimilitude, the girl gets to leave the system with the son.
Seriously? Nowhere in TRON cannon is it even hinted at that something like that would be possible. The laser-thingy (Does it ever get a name? Biological Transducer?) is able to translate people in and out of the system.
It can't work for programs, as they never had a physical form. If the interactions with the programs in the system are all creations in the minds of the Flynns', how does the laser- thingy even know what she's supposed to look like, and where does it get the raw ingredients to make her?
It's just silly. Of all the things in TRON: Legacy to call unrealistic, this is the only part that seems unbelievable.