If the runway is slippery and thrust reverse is used with autobrake, then there is no reliable way for the crew to judge the braking action. Any assessment of braking action depends on the decelerating systems used, their selection and setting, and the aircraft characteristics. The range of variables is so great that there is little or no value in the reports.
I disagree.
There is significant value in braking PIREPs especially when they report degraded braking. If a 747 reports Fair or Poor braking, you can be relatively confident that the braking is NOT good!
I have 17 years experience in the 747 Classic, -400, and -8. The -400 'stops on a dime', considering its size and weight. I can easily feel the brake releases when the antiskid is working hard. If I can feel those releases while using full reverse, the braking action is Fair AT BEST. If I can feel them when using Autobrakes 2, the braking action is definitely Poor.
As for this particular SAFO, I think it's more than a bit wishy-washy. Over the past few years I've read an increasing body of evidence that grooving and friction-enhancing surfaces may not be as good as once believed, especially after a few months or years of wear. I certainly am NOT a believer any more in the older teaching that a wet grooved runway is as good as a dry smooth runway.
When in doubt, ALWAYS use the more conservative performance calculations when dealing with wet runways.