Yeah, ACH is surprisingly not-unpleasant, at least relative to nightmares like X12 EDI with its implicit looping constructs and billions of companion guides that supercede random parts of the base spec.
X12 EDI, been there earned the badge, both on the generating and receiving sides. 837, 834, 835 and others in health care - fun! Positional format, with situational meaning... really it is a fascinating format.
i wrote the X12 EDI stuff for a major clearwater distributor... also wrote the ACH stuff that dumped someone a file and their job was to upload it via SFTP to the bank. so many badges.