I have a major problem with using Smart Collections: Performance.
I have a lot of Smart Collections that I use for cross-checking my submissions to Stock Agencies - probably more than 100 (and 40k images). They're grouped into small Collection Sets that I normally keep minimised.
When I open any one Collection Set to see the Smart Collections, my performance goes right through the floor. I'm running a really powerful PC, have a top-of-the-range GPU, a load of memory and everything is up-to-date. I've even reinstalled LR a few times.
With almost every operation, I get a blue spinning logo for about 10 seconds, then LR flashes off and on a couple of times (revealing the desktop), before I can do what I want for a few seconds, then the blue spinning logo and flashing again.
Dragging and dropping an image from one (normal) collection to another takes about 30 seconds, and usually takes three or four attempts to get it selected.
It is UNUSABLE in this state! Closing LR and reopening it solves the problem, until I then need to reference another Smart Collection.