1) multiple hundreds of dollars of cost.
2) it voids the warranty, so if anything goes wrong I can't get it fixed for free (I bought the extra length service contract).
3) it may be rendered inoperable by software updates at any time.
4) it is only useable for a certain amount of ink swaps, after that I guess your printer is dead. Sure, I may never do that many ink swaps in the lifetime of the printer but it still is an issue for me to buy something that is fundamentally limited.
I can't express my frustration with epson enough for their underhanded behavior. If I would have known about the inability to use whatever inks I would have wanted I would not have bought the p800. However, I am getting good results even from stock inks so retrospectively I feel like I have made some fairly tasty lemonade from the lemons epson gave me.