"HW rulez" mod (add-on) for HoMM3.

What imbalance are you talking about (even in the Conflux and Tower)? It's clear that the balance isn't in its favor when you consider how much effort it takes to find the Grail. You just want to dig it up for fun sometimes, but you can't.

Well, in the Tower, the entire map is visible to the mapmaker.
And in the Conflux, almost all magic is available. There are a few spells that I don't remember, but consider it like having 4 spellbooks.

In general, it's a very strong, overpowered item. The Capitol brings in 4000, and the Grail brings in a whopping 5000! It's overpowered! The Capitol is the primary goal in development. If you don't mind sacrificing some tempo, dig around, and find the Grail, you'll have 9000. With that income, no enemies are scary. It all pays off!
There's also a very overpowered artifact, the "Statue of the Legion" (maybe it should be banned too?). It halves the growth rate, which is very cool. Unlike some native secondary building, which is hard to build.

Conclusion