FWIW I worked with a guy who insisted on doing it this way and it was extremely twitchy.

Problems included various operating systems and network tools knocking in ways that iptables hates but a knock daemon will handle (e.g. sending two requests per port); making iptables rules unnecessarily complex and hard to audit and eventually conflicting with other things that interacted with iptables; and, once, accidentally getting the knock-related rules nuked by some other software and turning a simple login into a fun little puzzle at exactly the wrong time to be solving more puzzles.

We tested both approaches early on. A working knockd implementation cost less than an hour, including getting other users set up to interact with it. The iptables implementation he switched to shortly after ended up costing dozens of hours.