Yup, there were certainly some that would fail. It wasn't just the 351W that had the nylon gear, the loPo 302 had it as well, while the HO got the double roller. Later, ford upgraded the LoPo to the double roller, as my '89 Townie had one stock, but an earlier Vic an acquaintance had apart had the nylon teeth on the upper gear, and it was single roller. The double roller setups were pretty bomb proof though.
The 300 I6, despite being known for its durability, was an engine that had nylon timing gears for a period. They would also fail, that's what killed my '88 F-250 and why it ended up with a 302HO in it.