Known method, unknown figure
A review clause of this kind names a starting rent, an index, and the intervals at which the rent is recalculated — commonly every ten years. The clause is doing something quite reasonable in principle: holding the rent’s real value steady rather than letting inflation erode it. The difficulty is practical rather than philosophical.
Where the rent increases by reference to a recognised formula such as the retail prices index, it is difficult to predict with any certainty what the future ground rent will be. A doubling clause tells you the worst case at the outset. An index-linked clause tells you the method and leaves you to guess the inputs, over a term that may run for another century.
Read the clause rather than the summary of it. Index-linked clauses vary on points that change the answer considerably: whether there is a floor below which the rent cannot fall, whether there is a cap, whether an "upwards only" provision applies, and which index is named — RPI and CPI have diverged materially and for extended periods.