With some Split TCP schemes, a data packet can be acknowledged without first reaching its destination. Why is this a problem?

It is a problem with some split TCP scheme because it is a kind of violation of end-to-end TCP semantics. In fact, an acknowledgment originating from the wireless gateway may reach the sender before the corresponding data packet reaches its destination. In case, if the gateway crashes after the acknowledgment has been returned to the sender but before the data packet has reached the receiver, the sender will incorrectly assume that the packet has reached its destination safely. 
There is another issue with some split schemes is that wireless gateways face significant overhead as packets must undergo TCP processing twice.