I'm not 100% sure on that, but it would seem that kind of accusation is an attack on K-9 Officers' integrity and professionalism... which I would see more as rare- the vast majority of police are honest and have the integrity to not make up evidence. I wouldn't go nearly as far as to say "most" alerts by drug-sniffing K9s are planted based on cues by the handler. That's like saying "most vehicle searches are preformed on false pretenses by dishonest cops" which is simply not true. As far as evidence obtained by drug dogs- the SCOTUS ruled it's admissible in court- Florida V. Harris where "Justice Elena Kagan stated that the dog's certification and continued training are adequate indication of his reliability, and thus is sufficient to presume the dog's alert provides probable cause to search, using the "totality-of-the-circumstances" test per
Illinois v. Gates." A decision made Feb 19, 2013.