If it helps any, I have ran Mephiston into a Riptide before, mostly because his other target ran away at the end of shooting
He failed exactly 1 save and I saved 1 warp charge for force weapon. Dead riptide at I7, Meph swaggers into terrain for a 5+.
Thinking back on it, you're just about on averages with Meph vs. a Riptide. You get 6 attacks on the charge, if you hit all at 3+ you get to wound at 2+ with Sanguine Sword (assuming you didn't pop Unleash Rage, I think preferred enemy vs. Sword is about the same overall), if you manage to get 6 wounds on after those dice, not terribly difficult, then you should on average see a single failed armor save. And after passing one psychic test, you get 1 dead Riptide.
Otherwise the MC will hit on 5s wound on 4s and probably knock a wound of Meph, letting you try for that mystical 1 armor save again.
Anecdotal evidence aside, I personally like the BA/DA list. Its fast, has gatling guns, and embodies the Angels of Death. I think it would be more fun for me to play than the BA/SM.