My knowledge of the subject is fairly limited to my criminal law/procedure classes in law school and my brief stint with the Nashville District Attorney's office (as a clerk). There is no reluctance to use such evidence in court. In fact, cops and DAs love this stuff. As long as the evidence itself is credible (i.e. demonstrably not fraudulent or manufactured), there is no limitation of its use. They don't have to worry about a judge excluding the evidence on a technicality or bad procedure.
The key thing to remember is that the Constitution regulates governmental actors, not private citizens. The Constitution was designed to prevent the government (and the majority using governmental powers) from abusing the citizenry. Hence the 4th Amendment, which precludes the government from acting outside of due process (except for certain circumstances such as "hot pursuit" or preventing the destruction of evidence). There is no such limitation on private actors. It may be illegal (such as taping conversations) but unless the law barring the private actors from performing the conduct specifically provides, the evidence may be freely introduced.
As another example, did you know that if you are outside the jurisdiction of the court, you can be kidnapped, brought before the court, and the court will NOT dismiss the case on those grounds? Terrorists/drug lords/etc. have been successfully prosecuted in federal court after being literally kidnapped by military/LEO. The basic rule is the court doesn't care how you got before it, only that you are.