The good thing about runes reforged is that it does creates new theorycrafting, because it's new. Though, this is by no means something that you can attribute to the system itself, this is an aspect of any new mechanics put into the game but not even unique to new systems. Even if they had changed all the numbers around in the the only system, then you'd have tons of theorycrafting again.
There are two main issues with runes reforged. Power disparity exaggeration and longevity.
Yes it feels fresh and new now, but how long did it take riot to produce this system and can it stand on it's own as a dynamic and interesting system before it gets stale, requiring riot to undertake another massive rework. Well if we go off of history, then keystones didn't last long relative to the previous mastery system.
But even without using a historical case, just look at the design of the runes. Yes, there's a few options, but the optimal builds are relatively obvious, you KNOW which greater rune will be favored for which champion and for many classes it's almost universal which greater rune they pick.
The minor runes are a little less clear, but also not very difficult to decide on, once a large enough sample size determines the most powerful options.
Point being, this rune system while fresh, will stale very quickly.
The other point is the power disparity which is really what's likely to lead to the short lifespan of the system. The fact that the difference between an optimal selection and a non-optimal selection is so great, means that any champion which has inherent synergies with the existing selections will be inherently stronger than those who are incapable of maximizing them. I've already seen some people concerned, because how does this impact balancing of champions? Does, say
need to be buffed, simply because she can't effectively maximize the power of any of the runes which are designed for her role?
They've pigeonholed players and champions into 15 slots. Those slots are called greater runes.