tbh mhx actually kinda sux
Jack is basically better in most respects, but the pool of Gold assassins is so tiny that if you get lucky and get MHX before Jack, you shouldn't complain much. I mean the pool of gold assassins is basically MHX who is limited, and then Carmilla and Jack. We don't count Stheno because she sucks aside from being waifu. Then the next best is fucking Hassan, who while is major badass, is still a Bronze servant and could be godlike with higher stats.
For those of you looking for ideas on how to farm Scathach, here's my team to farm that Lv85 quest:
EDIT - I just picked up Artoria Star CE, so I replaced Mashu's CE with it.
EDIT 2
got a 2nd artoria star CE this time while farming Scathach. rofl