Ooooh, a topic of debate AND permission to build text walls. You asked for it buddy.
Firstly armor effects vs bosses. The slow debuff is just an idea for one aspect of the armor set bonus, it doesn't necessarily even have to work on bosses. It was more of an idea for holding hordes of enemies back a bit for survivability and because I didn't want to give arrows much more damage then they already have. As for mobility, I'm not saying the ranger has less than other classes, just that the need to use mobility renders the set bonuses useless on both vortex and shroomite. The only situation I or anyone I've played with has been able to use them effectively is when farming drops, and in that case you might as well send in the summoner who can do the same thing while afk. Giving the ranger different set bonuses isn't so much about making life easier as it is bringing them in line with the other classes late game.
As for debating the ranger struggle, it's a difficult thing to pin down. As DPS'ers Rangers should excel against bosses and it's certainly understandable that they'd be squishy to compensate. I would even say they do excel against most bosses, all if in normal difficulty. However, they just don't have any real survivability/dps against expert plantera and moonlord. They do a lot of damage during events, but they also die a lot if they don't make a cheaty hidey hole. Ranged is the most powerful against the mechanical bosses, but that's because of the OP Stormbow. I don't want to completely break the glass cannon concept, so I thought a slowness debuff or escape method as a set bonus would give ranged a touch of skill based survivability and create an interesting team dynamic. I would still also give ranged sets a little more armor, just a little.
I've seen people do that trick to the solar potato, in expert mode one of those chlorophyte shots always hits a Selenian that happens to randomly be jumping and the ranger ends up one-shotting himself with reflect damage. A universal problem, but ranged players die a lot faster to it.
Hmm, about where to add armor penetration. I think it will have to be in more than one place, partly because it's needed early on for use with the minishark, and partly because I feel that it has to scale a bit. When you get the minishark, the sharktooth necklace and a potion ought to be perfect, but late in the game enemies gain more armor at a level where simply reducing 5~8 total isn't enough despite the way armor is calculated with diminishing returns. I just now took a moment to think about where I'd put it in. I would say add JUST 3 armor penetration for ranged damage to the Ranger Potion so it helps early, but doesn't overdo it when stacking penetration, and add 5 penetration when using guns specifically to the rifle scope. "Adds 5 armor penetration and increased view range to guns". Of course keep the stat when upgrading to the sniper scope. This way expert mode guns get help without ruining normal mode or boosting bow/rocket damage too far. 13 Armor pen will knock the 30 damage lost per shot on a moonlord hand down to 20.25 lost per shot. Changes to the set bonuses of shroomite and vortex might make the sniper scope change unnecessary. Barring that, this is my suggestion.
Good talk.
Edit: And another thing, completely unrelated to the above. I HATE it when bosses despawn, either from being traveling out of their biome (especially the worm), getting too far away from you (plantera), or just spazzing out in general. I wish all bosses just went enraged instead and returned to normal once their requirements are met again. Quite often bosses just bug out and vanish on their own even under perfect conditions. It sucks.
