I've been told the numbers are fairly low, but thats improbable because they don't issue Customer Satisfaction Bulletins for things that don't often happen. You may be better off having one of the latest phaser designs due to your production date.
You would be surprised to find out just how low of a failure rate over expected it takes to make it less costly to just bite the bullet and spring for replacement. It’s singe digit percentages in some cases, depending on parts & labor cost, reputational risk, customer animosity etc.
As an example, I’d probably go with the 2017 - ’18 plastic oil pan, still not leaking on my truck 7 years on. The part is not that expensive and of course they will exhaust current inventory before using a revised pan, so you get the same part with the same intricate replacement procedure that few stealerships seem to be able to follow. This is where I think Ford had the problem - repeat failures because of poor workmanship.
Phasers I think were a combination of issues, where they didn’t fully appreciate the duty cycle problem, but, who’s to say there weren’t some early revisions that may have actually been defective? we don’t really know that. We now understand that flashing that duty cycle prevents further excessive wear. Either way, the initial diagnostic failures led to errant troubleshooting, and probably several hundreds of phaser repairs that may not have been needed were the duty cycle identified sooner. Maybe. Unlike the oil pan, where the proper procedure to get a good seal was well known from the jump, the phaser issue may not have been properly identified by Ford engineering. How many repeat phaser repairs did we have on FRF from ’17 - ’19? More than a few.
However, the phaser repair is labor intensive, and cost intensive, so I believe fewer of the phasers were needed to reach the CSP threshold - several hundreds in parts cost, plus the labor.
The failure rate is unpredictable, largely influenced by maintenance practices, and use case.