The biggest drawback is timing: you still need to survive long enough to cast it on turn two
In fact, Primeval Titan has been the face behind some of the most degenerate decks in Modern, starting with the Cloudpost deck that was prevalent in the initial days of the format, to the Red-Green Valakut deck, and then the Amulet Titan deck both in its degenerate form with Summer Bloom, and the more toned down version we see today with Dryad of the Ilysian Grove and Urzas Saga
And in the spirit of group hug, this does the trick
Look at TCG low, knock off 10 or 15 percent, and ask for that much in cash
Another factor that impacts the longevity of Pokmon card investments is supply
This certainly gets better as you get to the late game because the cost reduction applies to commander tax as well