I'm having trouble finding references for the tactical level codes that the Navajo Code replaced. I've only seen notices to machines that were easily cracked. Perhaps they are referring to the M-209? But official post war sources state that only 10-30% of an M-209 message was able to be cracked.
Any really good government documents would be highly appreciated.
As codes go, the Navajo code talk was much less secure than a system like the M-209 and was not really a replacement.
M-209 was a polyalphabetic cipher machine that could use a new key each day. Think of it as enigma-lite. The typical transcription speed was 20 letters a minute and the encrypted message was then sent out by morse code. The more messages you send with the same key the easier it gets to decode, but you're still probably safe for at least a day or two.
The Navajo code has only one key; you're not switching languages each day, although they swapped in new words from time to time. It only makes sense if you only use it for voice communication. If the Navajo messages were ever sent in morse code they would have been solved immediately. If it were used a lot of times, the enemy could start to assemble a phonetic lexicon.
The plus side of the Navajo code was that it was fast. It's great for things like enemy aircraft sightings or calling for artillery support when every second matters. In some situations it would have replaced unencrypted English speech, so in that sense it was more secure.