Unity constant update order?

I know unity does not call the Update function on a certain way unless I ask it to in the ‘Script Execution Order’, but can I rely on unity to update the game objects at the same order each time? (so if it chooses A over B, A.Update will always be called before B.Update)

Unless it is documented, I wouldn’t rely on it. I would expect minor changes to the scene hierarchy to be capable of disrupting the order between existing GameObjects in unpredictable ways.

If you really need to control the order in which methods are called on instances of a script attached to different gameobjects then consider registering these script instances with a manager object, in the order you want, and having that manager script define an Update method of its own that chains to some other method on the registered instances of your other script.