What assumptions (in terms of energy and speed) do we have in alpha decay?

Hi, this question is for my physics lab tutorial.

We mathematically determined the minimum distance of an alpha particle and uranium-234 during the alpha decay of plutonium 238. Assuming conservation of energy and momentum, we were also able to calculate the ''final'' speed of the alpha particle.

However a question asks that the numerical value of the final speed of the alpha particle cannot be correct because we neglected some factor and it would affect the final speed of the alpha particle.

My guess is that the electrons of a daughter nucleus have some effect on the alpha particles as like charges attract and/or conservation of energy doesn't hold because it isn't a closed system thus velocity is not constant.

The question is pretty much theory. So, I guess my main question is what factors affect the final speed of an alpha particle during alpha decay?