Okay, lets do this! For the sake of simplicity. TWFing build. 60% hit chance (AC16, +8 to hit; 0.86 with advantage, 0.75 for post-Vex weapon to hit), no magic items, no relevant feats, Dex 18. Assume we’re in a 1v1 duel against a noble of appropriate CR that picked a fight at a party, and there’s tables and chairs strewn about. The assassin is pretending to be a fellow noble, and the fey wanderer is an envoy from a fey court visiting.
Rogue (assassin) 9 TWF – The assassin wins initiative due to subclass. Has advantage. Start with Vex weapon, followed by Nick weapon. Uses BA for Steady Aim. Uses reaction to halve damage from enemy attack once. First turn involves dousing weapon with poison as a BA (potion use).
Normally I would also be using the Poison Cunning Strike, but that only applies defensively.
0.86(3.5+4)+0.744(3.5)+0.965(3.5*5)+0.86(3.5poison first turn only) + 9 assassin bonus= 25.85 + 12 1st-turn
Ranger (fey wanderer) 9 TWF – Barring feats, the enemy and ranger are roughly tied for initiative. Vex, Vex, Nick. BA is used for Hunter Marks, since we’re doing just raw damage and most of the Ranger good spells are AoEs or control. Furthermore, there’s a strong chance of the Ranger just… losing concentration on spells from being hit.
Of note is that the Noble Prodigy has Beguiling Strike and would proc the level 7 Beguiling Twist ability. A ready action allows one to ignore the Charmed effect for the most part, but its worth noting that the Fey Wanderer actively benefits from getting hit once.
0.6(3.5+4+3.5)+0.744(3.5+4+3.5)+0.744(3.5+4+3.5)+0.95(2.5) = 25.343
Note that if we had been able to maintain concentration with War Caster and took the Duel Wielding feat, that’d be another weapon attack for 8-ish damage.
R&R 54 TWF – This one is a bit more confusing. Same subclasses, but… there’s competition for BA. So, we’re going to be effectively using the same attack pattern as Fey Wanderer, but with a catch. The sneak attacks won’t proc on the first hit, since we won’t have Advantage by default. We can’t use Steady Aim, as that’s taken up by H.M. Accurately figuring out the math on that bit is beyond me, I’m afraid.
No using Cunning Actions or Uncanny Dodge for defense. No Expertise on two skills. Not that they’re relevant here, but it is worth noting that this hybrid is significantly less survival oriented than either pure class.
First turn advantage, but second+ does not.
0.6(3.5+4+3.5)+0.744(3.5+4+3.5)+0.744(3.5+4+3.5)+0.95(2.5) + 7*S.A.% = 30.39 + ~5.5 first turn
Okay, so the multi-class is doing about 5 more damage than either single class (except 1st turn assassin, which is 1.5 lower), HOWEVER. There is significant loss of utility and defensive options on the class in exchange. And a notable lack of AoEs from the full class Ranger.
EDIT - okay, so new math. Sneak attack only hits in two situations. You hit on the first attack and immediately on the second (if you miss, you lose advantage on third attack, thus ineligable) or you miss first, then hit the next two attacks. That's 0.6 * 0.86 * 1 + 0.4* 0.6 * 0.86? That's 72.24% hit chance, for an average of 5.05 damage.
The math only gets worse if we trade in TWF for a bow, because you only have ONE chance of hitting sneak attack on this build. That's about a 50% chance. And you don't get anything like Lightning Arrow, so I'm going to take another look at bow-rogue-ranger-mc and see how that shakes up... AFTER work.
Edit2- forgot assassin extra damage.