Maybe it's just me, but I've never had any problems like that with any type of system (hm, well, Secret Arts for Weapons of the Gods still fries my brain, but that's down to the way it's written, not the system itself), and I've never played with anyone who has in 30+ years of RPGing. If we did play with someone who couldn't cope with rolling high/low, 3d6, 1d100, 5d4+2d6, working out percentages etc,etc I think we'd probably give them some very strange looks! If they were rabid complainers about it I don't think they would last very long. It seems to me to be a more online theoretical complaint than a real one. TBH sometimes I feel that a completely unified system would be less interesting, as every roll would feel the same - I certainly found that with HeroQuest anyway.
WFRP scales everything to 1d100, but that then causes problems for large creatures - if a human can (even theoretically) get 100 STR, what do you do for bigger things?
If the 3d6 range is too near to zero at the bottom, you could always use 2d6+6 for humans, as per INT and SIZ.
cheers,
Mark