Without fear of sounding too generic of a reply.
I haven't experienced any personal issues with losing threat on a mob (not stating it's not happening, just isn't to me as far as I can tell), but I do agree it's getting to the point now where I have to monitor my combo splits more effectively and can foresee in the next patch or two with further content ilvl increases it'll be an issue as PLD's will be outscaled.