NS is more challenging to play well, because you need to learn to swap weapons (sometimes several times during same fight) and knowing precisely what poisons you put on which and where etc, plus you have no IP to rely on to save you, if you fk up, you vanish or die (more often than not die after vanish too vs competent opponent), while ranger can still give you a comeback with IP.
On the other hand if you don't camp CD's on Ranger and run into competent sins, you'll find yourself back at healer so fast that you won't know what happened.
The fact that you can self buff yourself with superior buffs to combi is irrelevant vs sins as you will be super debuffed and weakened all time (except when you get the opener for few swings).
Sins can climb tower / keeps, Ranger melee spec shouldn't ever get close (if you wanted to leech rps in zergs vs zerg / sieges, you would have made a sniper and not a melee anyway) and you can pick up lonely casters trying to hero mezz or bolt some poor soul downstairs
IMHO if you want to be a stealth bad$ss melee, make a szerk / mercfil (even with DS nerf it's still damn strong) and you should beat easily CS spec'd sins and have a good fight vs melee archers - tho with all due respect am not 100% certain how viable szerk / mercfil is on phoenix compared to OF live (I played both back then and had a blast tbh), so others could chime in and shed some light on it