Why not, is there something preventing me from digging a really deep hole and falling out the other end?
The UA forces would crush you against the bottom. This is, of course, assuming you magically dug through kilometers of rock and magma.
No it wouldn't. If the world was magically flat, and UA magically existed you would not be crushed. Your acceleration would already be 9.8 m\s, which would be one G. Emerging on the magical underside of the earth would not crush you as the magical UA would act on you the same as it does on the entire earth. You would experience the same effect as you were on the magical topside of the earth, which is a 9.8 m\s acceleration in the negative direction to earth.
The reason you would be pushed back to the earth on the magical underside is that you would be closer to the magical source of UA. And any force decreases or declines in a square to or from the source. Two times closer, four time stronger. Two time further away, four times weaker.
Congratulations, your statement actually proves that the earth is not magically flat. If your magical UA did exist, then anything and anyone on the magical topside of the magical flat earth would also be crushed they would feel the same force pushing against them. And no the magical UA would not lessen the force against them since the flat earth wiki already states that the magical UA curves around the earth to push the magical 32 mile sun and moon upwards. And again, if the magical UA pushed against the sun and moon, it's magical force would be slightly less than 9.8 m\s. Thus allowing the magical flat earth to catch up to the magical 32 mile diameter sun and moon. Given the age of the earth, this would have happened billions of years ago.
Disproving the magical flat earth crap is so easy, that a child with mental impairments could do it easily. I wonder what that says about magical tom and his magical fantasies?