I haven't seen an ACH risk assessment. I might start by enumerating ways we could expose ourselves to risk and how well we have mitigated those risks, e.g. only customers can be originators, Originator contracts contain all points identified by NACHA, no cross-border transactions, etc.
We work with the originator at the time the contract is in place to set a maximum amount per day. The loan officer has to approve this since this could turn into an unanticipated loan if the transfer is not funded. If a day's total transactions exceed the limit, the loan officer must approve the exception before it is processed. In the event that it isn't approved, the originator is contacted.
Don't know if this is what you needed but may be it will get you started.