Does a problem occur if multiple Updates access and execute a function in one script at the same time?

Does a problem occur if multiple Updates access and execute a function in one script at the same time?

Depends exactly what you mean. Normally no script execution in unity happens “at the same time” unless you are using threading. Unity will call update for each object, one by one every frame, not all at the same time. So if that is what you meant, no nothing unexpected should happen.

If you have a look at the Unity Order of Execution, you’ll see that the only parts of Unity that are multi-threaded are those in grey boxes with square corners.

What is not said on this page but is stated here is “the script execution order specifies that all scripts of one type are executed before all scripts of another type, regardless of which GameObject they are attached to”. That means that all the scripts of type “Awake” run before any other scripts. Then all OnEnable" scripts and so on.

In particular, none of the User Callback events (including Update, OnCollision etc) are multi-threaded so only one of them can execute at any one time. Therefore, it’s not possible to have multiple Updates execute other scripts at the same time.

You can write multi-threaded code if you really want to, using the Unity Jobs System and, of course, you have to worry about synchronisation and deadlock if you do so but that’s another topic…