You can’t identify an absent cadaver by the bark of a dog. If Shannon Matthews had been sold into white slavery never to be seen again a jury might conclude she’d been murdered in some grotty flat in Dewsbury, and was not out there somewhere in the big wide world waiting to be rescued.
Every situation is different. In the Suzanne Pilley case the circumstantial evidence is compelling even without the alerts by the cadaver dog in the office cellar and in the boot of David Gilroy's car. The fact that there were over one hundred miles unaccounted for in his trip up to the Scottish Highlands and that his car had three cracked coil springs and moss and soil clinging to its underside is imo pretty damning. Gilroy obviously told a pack of lies to the police investigators and subsequently refused to give evidence on his own behalf, an indicator of guilt in most situations.
IMO, honest people have nothing to fear by taking the stand.