Parallel ignition systems, anybody tried running one?

I've run the old $75 or less 6AL analog swap meet boxes with e-coil for years and never had a failure. Both cars have a 4 pin HEI installed that I can swap with a few connection changes to save the day and get home that I've never had to use.
When I ran the MSE boxes, it learned the hard way (walking home) to leave the OE set up wired in for just incase. Which always happened to me.