Care to briefly run me through their power combo?
It's clearly something that adds to assassinate ability where Assassins critical hit foes who haven't acted in the first round.
Bearing in mind assassin gets +d6 sneak attack every second level, Gloomstalker must get something more.
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[Edit] Looked it up myself.
Dread ambusher: level 3 ability, +Wis to initiative, extra attack with +d8 damage.
So level 6 Assassin has +4 initiative, +7 attack, weapon damage +4 dex, and +3d6 sneak attack
Level 3/3 Assassin-Gloomstalker has +5 initiative, +6 attack, 2x weapon damage +3 Dex, +d8 dread ambusher, and +2d6 sneak attack.
Considering the multiclass gains on average an extra doubled die on round 1, but falls behind rounds 2+, I think it's pretty balanced. Actually, it's kind of weak once you realise they're missing out on Expertise and Uncanny dodge.
I didn't define a level split, so that is on me, but people consider the build to be 'online' at Gloom5/Assassin3 :
Assassin makes everything a crit and for ease of math we'll assume all hits and do full damage to make things easy. We'll also assume we took the +2 Dex for the ASI, so +4 damage per attack. And we'll add hunter's mark for the rangers. Let's go with longbow so no worries about loading time and this will balance our damage die when we do the assassin numbers (as they can use a light crossbow).
Gloomstalker/Assassin gets 3 attacks that round plus a d8 to one of them. Math = 8d8 (64) +4d6 (24) +6d6 (36) +12 = 136
Assassin gets 1 attack and a better sneak attack = 2d8(16) + 8d6 (48) +4 = 68
Gloomstalker gets no sneak attack and no guaranteed crit = 4d8 (32) + 3d6 (18) + 12 = 62
So not too shabby at this point the Assassin is only half the damage. Now lets look at missing our first shot:
Multi - 6D8(48) + 4D6 (24) + 4d6 (24) + 4d6 (24) +8 = 94
Assassin - 0
Gloom - 3D8 (24) + 2d6 (12) 8 = 44
Big difference there, but the math isn't going to be half misses/hits except for maybe one specific enemy AC and I'm not about to do all that math.
Now turn over turn damage, all hits:
Multi - 2d8 (16) + 2d6 (12) + 2d6 (12) + 8 = 48
Assassin - 1d8 (8) + 4d6 (24) +4 = 32
Gloom - 2d8 (16) + 2d6 (12) 8 = 36
Now one miss:
Multi - 1d8 (8) + 2d6 (12) + 1d6 (6) +4 = 30
Assassin - 0
Gloom - 1d8 (8) + 1d6 (6) 4 = 18
So the 4-6 times is probably hyperbole, but it is 2-3 times the damage easily.
I'm sure someone out there has a more robust breakdown or has better optimization than my napkin math, but you can see that the damage difference is not insubstantial.
