Finish notes must not be in the middle or at the beginning in a 1/4 or above stream. At the end, finish streams may be allowed if there is a reasonable sound for it and have at least 2 notes before it that are the opposite note (For example, xxooxxooX, xxxxooooX, xoooxoooX, etc).As I remember xoooX is not allowed as discussed, and it's not listed in the entire rules. As I know it is also stated that streams with 1/4 finishes must have stream length 5 or above (why this is not stated in the rule?), so a better solution would be explicitly stating how the last 5 notes can be for the any streams with finishes at the end.
The reason behind banning inappropiate finishes is that musically they just don't make sense. Imagine something that ends with xoooX. The x in the front, as a highlighting point of the neighbourhood, defeat the purpose of having an even stronger note at the end. Similarly, something like xoxoX, xoxxO, etc., does not making sense for us to emphasize the last note with some previous note has the same weight.
Remember, we are not considering whether it is impossible to FC (under a general frame of mapping maps are always FC-able theoratically), but to see if they make sense with the song.