I've been reading a lot of actual battle reports and watching a lot of footage involving insurgents pitted against western forces on Afghanistan and Iraq.
A couple of things come to mind where the FoF rules don't seem to mesh with actual real life actions
Insurgents in FoF are present in large numbers. This might be fine for something like Mogadishu but doesn't seem to be the norm in Afghanistan/Iraq where Western forces outnumber the insurgents.
Insurgents really do rely on hit and run, ambushes etc. They use terrain well so Western forces usually struggle to find them.
They also seem to rely on range in Afghanistan (hence return of 7.62 mm battle rifles in DMR role to engage insurgents at long distances).
As such Western forces are "pinned" in a fashion – not psychologically pinned down but rather lack the initial intelligence to engage effectively at least initially (though snipers were increasingly used as battlefield ISR units to support situational awareness).
Then there's the IEDs, using civilian shields, using Western rules of engagement (eg dropping a weapon, running to next position knowing Westerners can't shoot unarmed troops and grabbing a new rifle) etc etc.
The result is this asymmetrical concept – a handful of insurgents can stall and pin down a much more numerically superior and better equipped force.
Now I'm not finding this in my FoF games where I often run out of insurgent models (and I have something like 70 insurgents) despite facing only a handful of westerners who then proceed to mow them down with insane accuracy (D6 v D8 or D10).
The Hidden/PID/Is That A Rock rules are meant to achieve some of above but it's not really showing in my games;
Hidden – well the models are on the table so only hidden on paper. The Western player knows exactly where the models are located so can plan accordingly. Hiding relocated units (especially those using Out of Contact Movement) is hard as one again needs a 4+ on a D6.
PID – somewhat of a joke as passed on a 4+ on a D8 or even worse D10. It's one of those "pray for bad dice" kinda rules.
Is That A Rock Or Muj- Hard to spring with D6 troops as this limits ambush zone to only 12 inches. Insurgents often ambush from much more stand off ranges.
Some potential solutions
Insurgents to count as D8 troop quality for defense and ambushes
Insurgent units to be fielded in smaller numbers (3-5 max) with fewer reinforcement
Consider utilizing blinds for hidden units
I also find the Leader rules are problematic – I've often had no units activate (they need a 4+) and it makes it purely random for Taliban player.
It plays more like Lone Survivor (over exaggerated even if still pretty amazing feat but boring games) or Seven Hours (work of fiction) than actual combat ops.