PliotAware will eventually become the conspicuity Betamax. It uses unlicensed frequencies so will never be certifiable.
FLARM is useful for sailplanes in close proximity with each other and with a few powered aircraft flying from the same site.
The CAA has already said that the preferred solution will be based on ADS-B. So the contender for the most likely future system is probably Sky Echo platform plus a display / audio warning system.
When I reviewed a number of conspicuity devices a year or so ago, the only one which was of any use was the Trig device which didn't need a display - and gave only genuine collision warnings. I was quite shocked at the 'panel gazing' of many of the GA pilots involved - VFR implies L00KOUT, not staring at some iToy!