It's not valid IMO. I once did some light research into how and how often a FISA warrant appears in a criminal prosecution. The most concise short answer is "never." "Rare" is technically correct, but doesn't give the right impression of exactly how rare it is.
I think LS's speculation is reasonable, just that when plugged into the cases and practical reality, it falls flat.
Prosecutors strongly tend to not use FISA in criminal cases precisely becuase doing so opens a can of worms, a can of worms that can nearly always be avoided.
On the Admissibility of FISA-Derived Evidence in Criminal Prosecutions
ThanQ - I am relieved. I thought he had some reasonable points; but not having any legal knowledge, I hoped I was wrong.
Much obliged.