On paper they blow old school springs away. Why aren't they used almost exclusively?

I have them on a vortec 350 with a gm hot cam, spun up to 5700 no issues.
using isky 8205 double springs on the big blocks though

some people love them, some hate them. all I know is I have what is left of a 496 stroker after the center of the retainer let go and dropped the valve. one of the problems as explained to me was if there is a spring failure there is a good chance of it knocking off the keepers where with double springs there is still tension on the retainer to keep it in place and the valve up. as for the one I have, Comp Cams had a big problem with retainer failures even though they down played it. I hear they have since fixed the problem but I have had so many comp failures the last few years that I wont even use their parts anymore.

They don't seem to be proven for BBC's.

SBC's is a non issue.

the bbc uses a much heavier valve than a sbc,the extra weight prohibits a beehive spring in a marine endurance engine.valve springs&retainers are not all created equally.

I guess I should have specified for big blocks. Fortunately on this forum it is assumed unless otherwise stated...

With the BBC valvetrain you don't mess with what works. On my old LSX builds we had good luck with Comp 918 beehives running street hyd roller grinds. Though as LSX cam lobe design went more radical we went back to the old faithful dual w dampner. I like the safety factor of a dual spring.