The non-blunt melee have Crit ratings. I think that they just wanted Strength to weigh heavily on Crits.Or just give unarmed and melee weapons crit dice like ranged weapons. You're already doing more damage and have a higher chance of landing crits by rolling better dice on your attack.I'll just go with the houserule that humans can only roll a d6 crit die against a vehicle with a blunt weapon.
(I kind of like the idea that a really strong replicant might punch through a vehicle.)
I like your idea that armor removes crits from covered areas. It's an easy houserule and makes the penalties more worth it. (It does make vehicles even more tricky, as they are entirely covered with their own armor... I guess just don't use the houserule for vehicles.) I want to see it all in play first, but I might use that houserule and mine about the d6 max Crit for humans vs vehicles.