What do you mean by parallel constructions? Is that where LE discovers evidence through extralegal means, then needs to rebuild the narrative through a legally valid chain? Could be, but then again there's probably a lot of TOR identities that are completely out of reach for LE, leaving them with only legal construction. Wouldn't you say?
I sometimes wonder about something, too: you know those "small" cases with huge human cost, like missing child, or murder in a backwoods area? I always imagine that classified capabilities could be used to solve them. The fact they are not, is painful, and I think must be "moral trauma" for LE/IC people involved. Even more so that they can't talk to anyone about it except their organizational therapists if then.
Tor seems to be a poster child of the "Nobody But Us"[1] principle the NSA likes so much: it's strong when used by American spooks, but weak when used against them. If a country developed body armor that's impervious to all rounds except their own special alloy rounds, their use and promotion of that armor is not evidence of its utter robustness.
I don't doubt a lot of darknet busts involve a lot of parallel construction - the intelligence community doesn't have to give detailed logs; summaries are enough (IP addresses, dates and times). This is before considering that the FBI is involved in both (counter) intelligence and law environment.
1. https://en.wikipedia.org/wiki/NOBUS