Because there is no reason to specify that they don't differ in power. The Nexus Event that generated the Infinite Ultron's timeline happened during Age of Ultron, this means that both sets of Stones were formed in the same way, expirienced the same things and were identical up until that point.
The main difference, which is why the Machine didn't worked, is that the Universes are incarnations of different Singularities that created different Universes, which is what makes them unique from each other.
Instead, you didn't brought up why we should believe that a set of Stones that was created in the same way than another and expirienced the same things should be notably weaker than an identical set, considering that even the Stones that we know were stated to be able to shread the Universe down to its last atom, a 3-A feat by itself.
Yes, they are Universal costants because they are the basis of the formation of the single Universes they are connected to. All the Timeline started from them, which means that they are constants of the Universes, and this is proven by the explanation of the Ancient One.
Now I really need to go to sleep, so if you have any further doubt I suggest you to bring this up in the actual CRT, or maybe some else will be able to answer your doubts. I just want to say that all of this was already discussed and agreed by many users, so I inserted that part because there was already a previous discussion.