It really depends on each individual airline. Somehow SQ's inventory for Krisflyer and PPS Club members are better than for other airlines' FQTV programmes.

I also cheat the system but checking on SQ's inventory myself first before making the appropriate booking through AC or UA or LH. That way, I know exactly which flight is available rather than AC, UA or LH requesting each flight that may not be available! Note that Business and First Class seats are much easier to come by than economy!