"I would guess they work best if the declarer doesn't know how to take advantage of the (disclosed) information…"
To decode hand parity signal information correctly one needs reference information from biding and play. Declarer often has a lot less hand pattern information available from biding and play than ...
I have tested the prisma signals (hand parity signals during play) in table. They do work but require quite a lot more thinking than ordinary count signals. Those signals give quicker the accurate count information. But I prefer using smith style encouragement of lead and suit preference instead.
As for ...
Slawinski like leads are nice to combine with smith like signal encouraging to discouraging lead even when defending trump contract.
I lead often J from Jxx just to make reading the lead easier. Even tough Jxx lead is less likely than xx lead.
I was just yesterday playing 2♥ in a bbo robot tournament.
Lead was ♥K that is obvious singleton from a robot. I ducked it. It is first trick end play but ...
Symmetric relay is based on Fibonacci like grouping for shapes. That can be made pretty close to optimal if 3NT is always the last safety level.
But pure Fibonacci grouping is suboptimal biding system. There is possible system design that puts some bids to have safety level at 3NT while ...
Declarer is likely to be short and dummy might hit down with 4 card heart. I would lead ten from KT8x but 9 lead from this is too small to matter. It won't pin declarers important spot card under it.
If declarer would be more likely to hold 4 ...
I like to open 1♦ and rebid 1NT forcing or 2NT ♥ fit to partners possible very weak 1M response.
But when playing more mainstream one level opening style this is close to 2♣ opening but I might forget to open 2♣ still.