Solarpunk Farming

Yields from this sort of grafting will almost always be lower than a tomato plant and a potato plant alone. A plant has a kinda "energy budget" for growth; It uses this much energy for vegetative growth, that much for roots, some more for fruit, etc. A tomato/potato graft is trying to "spend" large amounts of energy both growing large tomato fruits and growing tubers underground, so you end up with mediocre yields of both.

Even the tiny fruits a potato grows represent a small energy loss. Which (IMO) is why so many popular potato breeds don't produce true seed, you get just a little bit more potato that way.

Interestingly sometimes in potato breeding the opposite of this graft is used, a potato top on tomato roots. That way the potato greenery is flush with nutrients and can set a much heavier crop of fruits, allowing breeders to collect more seeds from their potential cross.