Dogs cannot be guaranteed to be 100% fail safe ... whatever they may be trained to sniff out ...
But do they need to be? Why is it always churned out...100%, fail safe? Ground penetrating radar isn't an exact science; the results need to be interpreted - the scan doesn't show a neatly packaged corpse under some soil, it shows subtle, nuanced changes in strata. Admittedly things have moved on, but it's not your panacea of 100%.
The dogs give an indication, an assistance to the investigation or a search; it's also not 100% and until we develop the technology to hold a conversation with a dog, assuming that dog has developed a level of comparable sentience, (preferably in human language, as opposed to dog) it will always be that way.
Look how far DNA has developed in the last 30 years, we're prosecuting way more now due to these advances, but it doesn't mean that we weren't using embryonic DNA analysis to catch bad guys 30 years ago - what percentage of 'fail safe' do you reckon that was?
If evidence is produced at a trial that proves 'beyond reasonable doubt', then the weight of
all evidence gathered, including specialist investigation techniques, is achieving the objective. It might be that, actually, in a trial, the evidence of an EVRD or CSI dog was a mere footnote; another layer to weigh against or for. It might be dismissed out of hand, or argued by a skilled barrister that it's inadmissable or irrelevant because it's 'not 100%'.
But why wouldn't you use it? It's a relatively common place, widely used investigation tool.
Putting myself in the McCann's shoes as best as one can without knowing the multitudinous factors imposing opun them at the time, if I was told that these dogs were scenting cadaver / blood in the apartment my little girl went missing from, I wouldn't be researching and castigating the dogs, I'd be absolutely hellbent on finding out what in the name of Zeus' cat is going on - because essentially it means that it's now way more likely something terrible happened to her prior to going missing. I'd be beside myself and I would be imploring them to do it again, extend the search, get more of those dogs, follow the trail. Anything.