Speaking as a scientist, and therefore from a purely scientific point of view, it IS totally unreliable. If you use the bog standard 28 day cycle, day 14 as ovulation, and all the appropriate CM changes within the cycle (and assuming they are in fact an accurate indicator of fertile periods), there's absolutely NO guarantee that you'll avoid pregnancy just by not DTD on the day you're ovulating or more fertile. Just like there's no guarantee you'll conceive if you DTD on the days you ARE 'fertile'.
Sperm lives and survives for up to 5 days in the uterus. And once an egg is released, it can survive for a couple of days. If you DTD 5 days before ovulation, which could well be possible as CM changes so quickly, then there's potential there for fewrtilisation. Likewise, if you DTD 2 days after ovulation, when you're technically home and dry (so to speak), then there's also potential for fertilisation there too.
There's also the fact that you subconsciously feel more in the mood for sex when you're most fertile, and less so when you're less/not fertile. Its natures way of helping procreation along.
Add in the fact that you'll be hard pushed to find many people with a 'normal' cycle. Everyone is soooo different, and what works for one person won't necessarily apply to another.
Also, why do you think all the Catholic families (more so from previous generations, but still in some families and regions/countries nowadays) have lots of children? Look up the Duggar family as an example of not using contraception/natural family planning. Its an eye-opener, thats for sure!
Sorry, thats turned out rather ranty. Hope it helps in some way though!